diff --git a/src/leo/class21/BobDie.java b/src/leo/class21/BobDie.java
new file mode 100644
index 0000000..ef53f31
--- /dev/null
+++ b/src/leo/class21/BobDie.java
@@ -0,0 +1,76 @@
+package leo.class21;
+
+/**
+ * @author Leo
+ * @ClassName BobDie
+ * @DATE 2021/1/14 2:39 下午
+ * @Description
+ *
+ * 给定5个参数，N，M，row，col，k
+ * 表示在N*M的区域上，醉汉Bob初始在(row,col)位置
+ * Bob一共要迈出k步，且每步都会等概率向上下左右四个方向走一个单位
+ * 任何时候Bob只要离开N*M的区域，就直接死亡
+ * 返回k步之后，Bob还在N*M的区域的概率
+ */
+public class BobDie {
+    static class Recursion {
+        public static double livePosibility(int n, int m, int row, int col, int k) {
+            if (row < 0 || col < 0 || row >= n || col >= m) {
+                return 0;
+            }
+            return (double)process(n, m, row, col, k)/Math.pow(4,k);
+        }
+
+        private static long process(int n, int m, int row, int col, int rest) {
+            if (row < 0 || col < 0 || row >= n || col >= m) {
+                return 0;
+            }
+            if (rest == 0) {
+                return 1;
+            }
+            long p1 = process(n, m, row, col + 1, rest - 1);
+            long p2 = process(n, m, row , col - 1, rest - 1);
+            long p3 = process(n, m, row + 1, col, rest - 1);
+            long p4 = process(n, m, row - 1, col, rest - 1);
+            return p1 + p2 + p3 + p4;
+        }
+    }
+    static class Dp {
+        public static double livePosibility(int n, int m, int row, int col, int k) {
+            if (row < 0 || col < 0 || row >= n || col >= m) {
+                return 0;
+            }
+            long[][][] dp = new long[n][m][k + 1];
+            for (int i = 0; i < n; i++) {
+                for (int j = 0; j < m; j++) {
+                    dp[i][j][0] = 1;
+                }
+            }
+            for (int rest = 1; rest <= k; rest++) {
+                for (int r = 0; r < n; r++) {
+                    for (int c = 0; c < m; c++) {
+                        dp[r][c][rest] = pick(dp,r,c+1,n,m,rest-1);
+                        dp[r][c][rest] += pick(dp,r,c-1,n,m,rest-1);
+                        dp[r][c][rest] += pick(dp,r+1,c,n,m,rest-1);
+                        dp[r][c][rest] += pick(dp,r-1,c,n,m,rest-1);
+                    }
+                }
+            }
+            return (double) dp[row][col][k]/Math.pow(4,k);
+        }
+
+        private static long pick(long[][][] dp, int r, int c, int n, int m, int rest) {
+
+            if (r < 0 || c < 0 || r >= n || c >= m) {
+                return 0;
+            }
+            return dp[r][c][rest];
+        }
+    }
+
+    public static void main(String[] args) {
+        System.out.println(Recursion.livePosibility(6, 6, 3, 3, 5));
+        System.out.println(Dp.livePosibility(6, 6, 3, 3, 5));
+    }
+
+}
diff --git a/src/leo/class21/CoinsWayEveryPaperDifferent.java b/src/leo/class21/CoinsWayEveryPaperDifferent.java
new file mode 100644
index 0000000..0f6ffdf
--- /dev/null
+++ b/src/leo/class21/CoinsWayEveryPaperDifferent.java
@@ -0,0 +1,131 @@
+package leo.class21;
+
+import class21.Code02_CoinsWayEveryPaperDifferent;
+
+/**
+ * @author Leo
+ * @ClassName CoinsWayEveryPaperDifferent
+ * @DATE 2021/1/13 4:46 下午
+ * @Description
+ * arr是货币数组，其中的值都是正数。再给定一个正数aim。
+ * 每个值都认为是一张货币，
+ * 即便是值相同的货币也认为每一张都是不同的，只有一张
+ * 返回组成aim的方法数
+ */
+public class CoinsWayEveryPaperDifferent {
+
+    static class Recursion {
+        public static int coinWays(int[] arr, int aim) {
+            return process(arr, aim, 0);
+        }
+
+        private static int process(int[] arr, int rest, int i) {
+            if (rest < 0) {
+                return 0;
+            }
+            if (i == arr.length) {
+                return rest == 0 ? 1 : 0;
+            }else{
+                return process(arr, rest, i + 1) + process(arr, rest - arr[i], i + 1);
+            }
+        }
+
+    }
+
+    static class Recursion1 {
+        public static int coinWays(int[] arr, int aim) {
+            if (arr == null || arr.length == 0) {
+                return aim == 0 ? 1 : 0;
+            }
+            return process(arr, aim, 0);
+        }
+
+        private static int process(int[] arr, int rest, int i) {
+            if (rest < 0) {
+                return 0;
+            }
+            if (i == arr.length) {
+                return rest == 0 ? 1 : 0;
+            }
+            return process(arr, rest, i + 1) + process(arr, rest - arr[i], i + 1);
+
+        }
+
+    }
+
+    static class dp1 {
+        public static int coinWays(int[] arr, int aim) {
+            if (arr == null || arr.length == 0) {
+                return aim == 0 ? 1 : 0;
+            }
+            int n = arr.length;
+            int[][] dp = new int[n + 1][aim + 1];
+            dp[n][0] = 1;
+            for (int i = n - 1; i >= 0; i--) {
+                for (int rest = 0; rest <= aim; rest++) {
+                    dp[i][rest] = dp[i + 1][rest] + (rest - arr[i] >= 0 ? dp[i + 1][rest - arr[i]] : 0);
+                }
+            }
+
+            return dp[0][aim];
+        }
+    }
+
+    static class Dp {
+        public static int coinWays(int[] arr, int aim) {
+            if (arr == null || arr.length == 0) {
+                return aim == 0 ? 1 : 0;
+            }
+            int n = arr.length;
+            int[][] dp = new int[n + 1][aim + 1];
+            dp[n][0] = 1;
+            for (int i = n - 1; i >= 0; i--) {
+                for (int rest = 0; rest <= aim; rest++) {
+                    dp[i][rest] = dp[i + 1][rest] + (rest - arr[i] >= 0 ? dp[i + 1][rest - arr[i]] : 0);
+                }
+            }
+            return dp[0][aim];
+        }
+    }
+    // 为了测试
+    public static int[] randomArray(int maxLen, int maxValue) {
+        int N = (int) (Math.random() * maxLen);
+        int[] arr = new int[N];
+        for (int i = 0; i < N; i++) {
+            arr[i] = (int) (Math.random() * maxValue) + 1;
+        }
+        return arr;
+    }
+
+    // 为了测试
+    public static void printArray(int[] arr) {
+        for (int i = 0; i < arr.length; i++) {
+            System.out.print(arr[i] + " ");
+        }
+        System.out.println();
+    }
+
+    // 为了测试
+    public static void main(String[] args) {
+        int maxLen = 20;
+        int maxValue = 30;
+        int testTime = 1000000;
+        System.out.println("测试开始");
+        for (int i = 0; i < testTime; i++) {
+            int[] arr = randomArray(maxLen, maxValue);
+            int aim = (int) (Math.random() * maxValue);
+            int ans1 = Recursion1.coinWays(arr, aim);
+            int ans2 = dp1.coinWays(arr, aim);
+            if (ans1 != ans2) {
+                System.out.println("Oops!");
+                printArray(arr);
+                System.out.println(aim);
+                System.out.println(ans1);
+                System.out.println(ans2);
+                break;
+            }
+        }
+        System.out.println("测试结束");
+    }
+
+}
diff --git a/src/leo/class21/CoinsWayNoLimit.java b/src/leo/class21/CoinsWayNoLimit.java
new file mode 100644
index 0000000..440747e
--- /dev/null
+++ b/src/leo/class21/CoinsWayNoLimit.java
@@ -0,0 +1,163 @@
+package leo.class21;
+
+/**
+ * @author Leo
+ * @ClassName CoinsWayNoLimit
+ * @DATE 2021/1/13 5:21 下午
+ * @Description
+ *
+ * arr是面值数组，其中的值都是正数且没有重复。再给定一个正数aim。
+ * 每个值都认为是一种面值，且认为张数是无限的。
+ * 返回组成aim的方法数
+ */
+public class CoinsWayNoLimit {
+
+    static class Recursion {
+        public static int coinWays(int[] arr, int aim) {
+            if (arr == null || arr.length == 0 || aim == 0) {
+                return 0;
+            }
+            return process(arr, 0, aim);
+        }
+
+        private static int process(int[] arr, int i, int rest) {
+            if (i == arr.length) {
+                return rest == 0 ? 1 : 0;
+            }
+            int ans = 0;
+            for (int z = 0; z * arr[i] <= rest; z++) {
+                ans += process(arr, i + 1, rest - z * arr[i]);
+            }
+            return ans;
+        }
+    }
+
+    static class Recursion1 {
+        public static int coinWays(int[] arr, int aim) {
+            if (arr == null || arr.length == 0) {
+                return aim == 0 ? 1 : 0;
+            }
+            return process(arr, aim, 0);
+        }
+
+        private static int process(int[] arr, int rest, int i) {
+            if (i == arr.length) {
+                return rest == 0 ? 1 : 0;
+            }
+            int ans = 0;
+            for (int z = 0; z * arr[i] <= rest; z++) {
+                ans += process(arr, rest - z*arr[i], i + 1);
+            }
+            return ans;
+        }
+    }
+
+    static class Dp1 {
+        public static int coinWays(int[] arr, int aim) {
+            if (arr == null || arr.length == 0) {
+                return aim == 0 ? 1 : 0;
+            }
+            int n = arr.length;
+            int[][] dp = new int[n + 1][aim + 1];
+            dp[n][0] = 1;
+            for (int i = n-1; i >= 0; i--) {
+                for (int rest = 0; rest <= aim; rest++) {
+                    int ans = 0;
+                    for (int z = 0; z * arr[i] <= rest; z++) {
+                        ans += dp[i + 1][rest - z * arr[i]];
+                    }
+                    dp[i][rest] = ans;
+                }
+            }
+            return dp[0][aim];
+        }
+    }
+    static class Dp {
+        public static int coinWays(int[] arr, int aim) {
+            if (arr == null || arr.length == 0) {
+                return aim == 0 ? 1 : 0;
+            }
+            int n = arr.length;
+            int[][] dp = new int[n + 1][aim + 1];
+            dp[n][0] = 1;
+            for (int i = n - 1; i >= 0; i--) {
+                for (int rest = 0; rest <= aim; rest++) {
+                    int ans = 0;
+                    for (int z = 0; z * arr[i] <= rest; z++) {
+                        ans += dp[i + 1][rest - z * arr[i]];
+                    }
+                    dp[i][rest] = ans;
+                }
+            }
+
+            return dp[0][aim];
+        }
+    }
+
+    /**
+     * 观察严格以来表结构,简化枚举过程
+     */
+    static class OptimizationDp {
+        public static int coinWays(int[] arr, int aim) {
+            if (arr == null || arr.length == 0) {
+                return aim == 0 ? 1 : 0;
+            }
+            int n = arr.length;
+            int[][] dp = new int[n + 1][aim + 1];
+            dp[n][0] = 1;
+            for (int i = n - 1; i >= 0; i--) {
+                for (int rest = 0; rest <= aim; rest++) {
+                    dp[i][rest] = dp[i + 1][rest];
+                    if (rest - arr[i] >= 0) {
+                        dp[i][rest] += dp[i][rest - arr[i]];
+                    }
+                }
+            }
+
+            return dp[0][aim];
+        }
+    }
+
+
+    // 为了测试
+    public static int[] randomArray(int maxLen, int maxValue) {
+        int N = (int) (Math.random() * maxLen);
+        int[] arr = new int[N];
+        for (int i = 0; i < N; i++) {
+            arr[i] = (int) (Math.random() * maxValue) + 1;
+        }
+        return arr;
+    }
+
+    // 为了测试
+    public static void printArray(int[] arr) {
+        for (int i = 0; i < arr.length; i++) {
+            System.out.print(arr[i] + " ");
+        }
+        System.out.println();
+    }
+
+    // 为了测试
+    public static void main(String[] args) {
+        int maxLen = 20;
+        int maxValue = 30;
+        int testTime = 100000;
+        System.out.println("测试开始");
+        for (int i = 0; i < testTime; i++) {
+            int[] arr = randomArray(maxLen, maxValue);
+            int aim = (int) (Math.random() * maxValue);
+            int ans1 = OptimizationDp.coinWays(arr, aim);
+            int ans2 = Dp.coinWays(arr, aim);
+            if (ans1 != ans2) {
+                System.out.println("Oops!");
+                printArray(arr);
+                System.out.println(aim);
+                System.out.println(ans1);
+                System.out.println(ans2);
+                break;
+            }
+        }
+        System.out.println("测试结束");
+    }
+
+}
diff --git a/src/leo/class21/CoinsWaySameValueSamePapper.java b/src/leo/class21/CoinsWaySameValueSamePapper.java
new file mode 100644
index 0000000..5f94594
--- /dev/null
+++ b/src/leo/class21/CoinsWaySameValueSamePapper.java
@@ -0,0 +1,223 @@
+package leo.class21;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * @author Leo
+ * @ClassName CoinsWaySameValueSamePapper
+ * @DATE 2021/1/14 11:07 上午
+ * @Description
+ *
+ * arr是货币数组，其中的值都是正数。再给定一个正数aim。
+ * 每个值都认为是一张货币，
+ * 认为值相同的货币没有任何不同，
+ * 返回组成aim的方法数
+ */
+public class CoinsWaySameValueSamePapper {
+
+    static class Recursion{
+        static class Info {
+            int[] count;
+            int[] coins;
+            public Info(int[] coins,int[] count){
+                this.coins = coins;
+                this.count = count;
+            }
+        }
+        private static Info getInfo(int[] arr) {
+            Map<Integer, Integer> map = new HashMap<>();
+            for (int i = 0; i < arr.length; i++) {
+                if (map.containsKey(arr[i])) {
+                    map.put(arr[i], map.get(arr[i]) + 1);
+                }else{
+                    map.put(arr[i], 1);
+                }
+            }
+            int[] count = new int[map.size()];
+            int[] coins = new int[map.size()];
+            int i = 0;
+            for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
+                coins[i] = entry.getKey();
+                count[i++] = entry.getValue();
+            }
+            return new Info(coins, count);
+        }
+
+        public static int coinsWay(int[] arr, int aim) {
+            if (arr == null || arr.length == 0) {
+                return aim == 0 ? 1 : 0;
+            }
+            Info info = getInfo(arr);
+            return process(info.coins, info.count, aim, 0);
+        }
+
+        private static int process(int[] coins, int[] count, int rest, int i) {
+            if (rest < 0) {
+                return 0;
+            }
+            if (i == coins.length) {
+                return rest == 0 ? 1 : 0;
+            }
+            int ans = 0;
+            for (int z = 0; coins[i] * z <= rest && z <= count[i]; z++) {
+                ans += process(coins, count, rest - z * coins[i], i + 1);
+            }
+            return ans;
+        }
+    }
+
+    static class Dp {
+        static class Info {
+            int[] coins;
+            int[] count;
+
+            public Info(int[] coins, int[] count) {
+                this.coins = coins;
+                this.count = count;
+            }
+        }
+        private static Info getInfo(int[] arr) {
+            Map<Integer, Integer> map = new HashMap<>();
+            for (int i = 0; i < arr.length; i++) {
+                if (map.containsKey(arr[i])) {
+                    map.put(arr[i], map.get(arr[i]) + 1);
+                }else{
+                    map.put(arr[i], 1);
+                }
+            }
+            int[] coins = new int[map.size()];
+            int[] count = new int[map.size()];
+            int i = 0;
+            for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
+                coins[i] = entry.getKey();
+                count[i++] = entry.getValue();
+            }
+            return new Info(coins, count);
+        }
+
+        public static int coinsWay(int[] arr, int aim) {
+            if (arr == null || arr.length == 0) {
+                return aim == 0 ? 1 : 0;
+            }
+            Info info = getInfo(arr);
+            int n = info.coins.length;
+            int[] coins = info.coins;
+            int[] count = info.count;
+            int[][] dp = new int[n + 1][aim + 1];
+            dp[n][0] = 1;
+            for (int i = n - 1; i >= 0; i--) {
+                for (int rest = 0; rest <= aim; rest++) {
+                    int ans = 0;
+                    for (int z = 0; coins[i] * z <= rest && z <= count[i]; z++) {
+                        ans += dp[i + 1][rest - z * coins[i]];
+                    }
+                    dp[i][rest] = ans;
+                }
+            }
+            return dp[0][aim];
+        }
+
+    }
+
+    /**
+     * 观察严格以来表结构,简化枚举过程
+     */
+    static class OptimizationDp {
+        static class Info{
+            int[] coins;
+            int[] count;
+
+            public Info(int[] coins, int[] count) {
+                this.coins = coins;
+                this.count = count;
+            }
+        }
+
+        private static Info getInfo(int[] arr) {
+            Map<Integer, Integer> map = new HashMap<>();
+            for (int i = 0; i < arr.length; i++) {
+                if (map.containsKey(arr[i])) {
+                    map.put(arr[i], map.get(arr[i]) + 1);
+                }else{
+                    map.put(arr[i], 1);
+                }
+            }
+            int[] coins = new int[map.size()];
+            int[] count = new int[map.size()];
+            int i = 0;
+            for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
+                coins[i] = entry.getKey();
+                count[i++] = entry.getValue();
+            }
+            return new Info(coins, count);
+        }
+
+
+        public static int coinsWay(int[] arr, int aim) {
+            if (arr == null || arr.length == 0) {
+                return aim == 0 ? 1 : 0;
+            }
+            Info info = getInfo(arr);
+            int[] coins = info.coins;
+            int[] count = info.count;
+            int n = coins.length;
+            int[][] dp = new int[n + 1][aim + 1];
+            dp[n][0] = 1;
+            for (int i = n - 1; i >= 0; i--) {
+                for (int rest = 0; rest <= aim; rest++) {
+                    dp[i][rest] = dp[i + 1][rest];
+                    if (rest - coins[i] >= 0) {
+                        dp[i][rest] += dp[i][rest - coins[i]];
+                    }
+                    if (rest - coins[i] * (count[i] + 1) >= 0) {
+                        dp[i][rest] -= dp[i + 1][rest - coins[i] * (count[i] + 1)];
+                    }
+                }
+            }
+
+            return dp[0][aim];
+        }
+    }
+
+    // 为了测试
+    public static int[] randomArray(int maxLen, int maxValue) {
+        int N = (int) (Math.random() * maxLen);
+        int[] arr = new int[N];
+        for (int i = 0; i < N; i++) {
+            arr[i] = (int) (Math.random() * maxValue) + 1;
+        }
+        return arr;
+    }
+
+    // 为了测试
+    public static void printArray(int[] arr) {
+        for (int i = 0; i < arr.length; i++) {
+            System.out.print(arr[i] + " ");
+        }
+        System.out.println();
+    }
+
+    // 为了测试
+    public static void main(String[] args) {
+        int maxLen = 10;
+        int maxValue = 20;
+        int testTime = 10000;
+        System.out.println("测试开始");
+        for (int i = 0; i < testTime; i++) {
+            int[] arr = randomArray(maxLen, maxValue);
+            int aim = (int) (Math.random() * maxValue);
+            int ans1 = OptimizationDp.coinsWay(arr, aim);
+            int ans2 = Dp.coinsWay(arr, aim);
+            if (ans1 != ans2 ) {
+                System.out.println("Oops!");
+                printArray(arr);
+                System.out.println(aim);
+                System.out.println(ans1);
+                System.out.println(ans2);
+                break;
+            }
+        }
+        System.out.println("测试结束");
+    }
+}
diff --git a/src/leo/class21/MinPathSum.java b/src/leo/class21/MinPathSum.java
new file mode 100644
index 0000000..2fccfee
--- /dev/null
+++ b/src/leo/class21/MinPathSum.java
@@ -0,0 +1,210 @@
+package leo.class21;
+
+import class21.Code01_MinPathSum;
+
+/**
+ * @author Leo
+ * @ClassName MinPathSum
+ * @DATE 2021/1/13 2:15 下午
+ * @Description
+ * 给定一个二维数组matrix，一个人必须从左上角出发，最后到达右下角
+ * 沿途只可以向下或者向右走，沿途的数字都累加就是距离累加和
+ * 返回最小距离累加和
+ */
+public class MinPathSum {
+
+    static class Recursion {
+        public static int minPathSum(int[][] arr) {
+            if (arr == null || arr.length == 0 || arr[0] == null || arr[0].length == 0) {
+                return 0;
+            }
+            return process(arr, 0, 0);
+        }
+
+        private static int process(int[][] arr, int r, int l) {
+            int min = arr[r][l];
+            if (r == arr.length-1 && l == arr[0].length-1) {
+                return min;
+            } else if (r == arr.length-1 && l < arr[0].length-1) {
+                return min + process(arr, r, l + 1);
+            } else if (r < arr.length-1 && l == arr[0].length-1) {
+                return min + process(arr, r + 1, l);
+            }else {
+                min += Math.min(process(arr, r + 1, l), process(arr, r, l + 1));
+                return min;
+            }
+
+        }
+
+    }
+
+    static class Recursion1 {
+        public static int minPathSum(int[][] arr) {
+            if (arr == null || arr.length == 0 || arr[0] == null || arr[0].length == 0) {
+                return 0;
+            }
+            return process(arr, arr.length - 1, arr[0].length - 1);
+        }
+
+        private static int process(int[][] arr, int row, int col) {
+            int min = arr[row][col];
+            if (row == 0 && col == 0) {
+                return min;
+            } else if (row == 0 && col > 0) {
+                return min + process(arr, row, col - 1);
+            } else if (row > 0 && col == 0) {
+                return min + process(arr, row - 1, col);
+            }else{
+                return min + Math.min(process(arr, row - 1, col), process(arr, row, col - 1));
+            }
+
+        }
+    }
+
+    static class Dp {
+        public static int minPathSum(int[][] arr) {
+            if (arr == null || arr.length == 0 || arr[0] == null || arr[0].length == 0) {
+                return 0;
+            }
+            int r = arr.length;
+            int l = arr[0].length;
+            int[][] dp = new int[r][l];
+            dp[r - 1][l - 1] = arr[r-1][l-1];
+            for (int i = r - 2; i >= 0; i--) {
+                dp[i][l-1] = arr[i][l-1] + dp[i + 1][l-1];
+            }
+            for (int i = l - 2; i >= 0; i--) {
+                dp[r-1][i] = arr[r-1][i] + dp[r-1][i + 1];
+            }
+            for (int i = r - 2; i >= 0; i--) {
+                for (int j = l - 2; j >= 0; j--) {
+                    dp[i][j] = arr[i][j] + Math.min(dp[i + 1][j], dp[i][j + 1]);
+                }
+            }
+            return dp[0][0];
+        }
+    }
+
+    static class Dp1 {
+        public static int minPathSum(int[][] arr) {
+            if (arr == null || arr.length == 0 || arr[0] == null || arr[0].length == 0) {
+                return 0;
+            }
+            int row = arr.length;
+            int col = arr[0].length;
+            int[][] dp = new int[row][col];
+            dp[0][0] = arr[0][0];
+            for (int i = 1; i < row; i++) {
+                dp[i][0] = arr[i][0] + dp[i - 1][0];
+            }
+            for (int i = 1; i < col; i++) {
+                dp[0][i] = arr[0][i] + dp[0][i - 1];
+            }
+            for (int r = 1; r < row; r++) {
+                for (int c = 1; c < col; c++) {
+                    dp[r][c] = arr[r][c] + Math.min(dp[r - 1][c], dp[r][c - 1]);
+                }
+            }
+            return dp[row - 1][col - 1];
+        }
+    }
+
+    static class OptimizationDp1 {
+        public static int minPathSum(int[][] arr) {
+            if (arr == null || arr.length == 0 || arr[0] == null || arr[0].length == 0) {
+                return 0;
+            }
+            int row = arr.length;
+            int col = arr[0].length;
+            int[] dp = new int[col];
+            dp[0] = arr[0][0];
+            for (int i = 1; i < col; i++) {
+                dp[i] = arr[0][i] + dp[i - 1];
+            }
+            for (int r = 1; r < row; r++) {
+                dp[0] += arr[r][0];
+                for (int c = 1; c < col; c++) {
+                    dp[c] = arr[r][c] + Math.min(dp[c - 1], dp[c]);
+                }
+            }
+            return dp[col - 1];
+        }
+    }
+
+
+    static class OptimizationDp {
+        public static int minPathSum(int[][] arr) {
+            if (arr == null || arr.length == 0 || arr[0] == null || arr[0].length == 0) {
+                return 0;
+            }
+            int r = arr.length;
+            int c = arr[0].length;
+            int[] dp = new int[c];
+            dp[c-1] = arr[r - 1][c - 1];
+            for (int i = c - 2; i >= 0; i--) {
+                dp[i] = arr[r - 1][i] + dp[i + 1];
+            }
+            for (int i = r - 2; i >= 0; i--) {
+                dp[c - 1] += arr[i][c - 1];
+                for (int j = c - 2; j >= 0; j--) {
+                    dp[j] = arr[i][j] + Math.min(dp[j + 1], dp[j]);
+                }
+            }
+
+            return dp[0];
+        }
+
+
+    }
+    // for test
+    public static int[][] generateRandomMatrix(int rowSize, int colSize) {
+        if (rowSize < 0 || colSize < 0) {
+            return null;
+        }
+        int[][] result = new int[rowSize][colSize];
+        for (int i = 0; i != result.length; i++) {
+            for (int j = 0; j != result[0].length; j++) {
+                result[i][j] = (int) (Math.random() * 10);
+            }
+        }
+        return result;
+    }
+
+    // for test
+    public static void printMatrix(int[][] matrix) {
+        for (int i = 0; i != matrix.length; i++) {
+            for (int j = 0; j != matrix[0].length; j++) {
+                System.out.print(matrix[i][j] + " ");
+            }
+            System.out.println();
+        }
+    }
+
+    public static void main(String[] args) {
+         int[][] m = generateRandomMatrix(4, 4);
+       /* int[][] m = {
+                {7, 2, 8, 4},
+                {1, 2, 0, 8},
+                {7, 5, 1, 6},
+                {0, 7, 0, 7}
+        };*/
+        System.out.println("start");
+
+        for (int i = 0; i < 10000; i++) {
+            int recursion = Recursion1.minPathSum(m);
+            int dp = Dp.minPathSum(m);
+            int dp1 = Dp1.minPathSum(m);
+            int opt1 = OptimizationDp1.minPathSum(m);
+            int opt = OptimizationDp.minPathSum(m);
+            if (opt1 != opt) {
+                System.out.println("====");
+                printMatrix(m);
+                System.out.println(opt);
+                System.out.println(opt1);
+                break;
+            }
+        }
+        System.out.println("end");
+
+    }
+}
diff --git a/src/leo/remark.md b/src/leo/remark.md
new file mode 100644
index 0000000..ab930cb
--- /dev/null
+++ b/src/leo/remark.md
@@ -0,0 +1,13 @@
+## dp
+
+- 业务限制模型,可变参数不能直观的得到变换范围,需要把可变参数估出来,
+- 样本对应模式,两个参数就是下标,明确知道两个参数的变化范围,只讨论当前的结尾如何组织可能性
+- 范围尝试模型,LR两个参数可变参数,左下半区没用,先求对角线或上一个对角线,,只用上半区,特别在意开头和结尾.
+- 从左往右模型,明确知道可变参数的最大值
+
+**如果一个格子没有枚举行为,只依赖有限的几个格子,记忆化搜索和严格表结构的答案(时间复杂度)同样的好**
+**如果有枚举行为,需要搞出严格表结构继续优化**
+## String
+
+- 子序列可以不连续
+- 子串必须连续