diff --git a/src/class18/Code01_RobotWalk.java b/src/class18/Code01_RobotWalk.java
index f86fe3b..f9ee3f2 100644
--- a/src/class18/Code01_RobotWalk.java
+++ b/src/class18/Code01_RobotWalk.java
@@ -2,148 +2,93 @@ package class18;
 
 public class Code01_RobotWalk {
 
-	public static int ways1(int N, int M, int K, int P) {
-		if (N < 2 || K < 1 || M < 1 || M > N || P < 1 || P > N) {
-			return 0;
+	public static int ways1(int N, int start, int aim, int K) {
+		if (N < 2 || start < 1 || start > N || aim < 1 || aim > N || K < 1) {
+			return -1;
 		}
-		return walk(N, M, K, P);
+		return process1(start, K, aim, N);
 	}
 
-	public static int walk(int N, int cur, int rest, int P) {
-		if (rest == 0) {
-			return cur == P ? 1 : 0;
-		}
-		if (cur == 1) {
-			return walk(N, 2, rest - 1, P);
-		}
-		if (cur == N) {
-			return walk(N, N - 1, rest - 1, P);
-		}
-		return walk(N, cur + 1, rest - 1, P) + walk(N, cur - 1, rest - 1, P);
+	// 机器人当前来到的位置是cur，
+	// 机器人还有rest步需要去走，
+	// 最终的目标是aim，
+	// 有哪些位置？1~N
+	// 返回：机器人从cur出发，走过rest步之后，最终停在aim的方法数，是多少？
+	public static int process1(int cur, int rest, int aim, int N) {
+		if (rest == 0) { // 如果已经不需要走了，走完了！
+			return cur == aim ? 1 : 0;
+		}
+		// (cur, rest)
+		if (cur == 1) { // 1 -> 2
+			return process1(2, rest - 1, aim, N);
+		}
+		// (cur, rest)
+		if (cur == N) { // N-1 <- N
+			return process1(N - 1, rest - 1, aim, N);
+		}
+		// (cur, rest)
+		return process1(cur - 1, rest - 1, aim, N) + process1(cur + 1, rest - 1, aim, N);
 	}
 
-	public static int waysCache(int N, int M, int K, int P) {
-		if (N < 2 || K < 1 || M < 1 || M > N || P < 1 || P > N) {
-			return 0;
+	public static int ways2(int N, int start, int aim, int K) {
+		if (N < 2 || start < 1 || start > N || aim < 1 || aim > N || K < 1) {
+			return -1;
 		}
 		int[][] dp = new int[N + 1][K + 1];
-		for (int row = 0; row <= N; row++) {
-			for (int col = 0; col <= K; col++) {
-				dp[row][col] = -1;
+		for (int i = 0; i <= N; i++) {
+			for (int j = 0; j <= K; j++) {
+				dp[i][j] = -1;
 			}
 		}
-		return walkCache(N, M, K, P, dp);
+		// dp就是缓存表
+		// dp[cur][rest] == -1 -> process1(cur, rest)之前没算过！
+		// dp[cur][rest] != -1 -> process1(cur, rest)之前算过！返回值，dp[cur][rest]
+		// N+1 * K+1
+		return process2(start, K, aim, N, dp);
 	}
 
-	public static int walkCache(int N, int cur, int rest, int P, int[][] dp) {
+	// cur 范: 1 ~ N
+	// rest 范：0 ~ K
+	public static int process2(int cur, int rest, int aim, int N, int[][] dp) {
 		if (dp[cur][rest] != -1) {
 			return dp[cur][rest];
 		}
+		// 之前没算过！
+		int ans = 0;
 		if (rest == 0) {
-			dp[cur][rest] = cur == P ? 1 : 0;
-			return dp[cur][rest];
-		}
-		if (cur == 1) {
-			dp[cur][rest] = walkCache(N, 2, rest - 1, P, dp);
-			return dp[cur][rest];
-		}
-		if (cur == N) {
-			dp[cur][rest] = walkCache(N, N - 1, rest - 1, P, dp);
-			return dp[cur][rest];
-		}
-		dp[cur][rest] = walkCache(N, cur + 1, rest - 1, P, dp) + walkCache(N, cur - 1, rest - 1, P, dp);
-		return dp[cur][rest];
-	}
-
-	public static int ways2(int N, int M, int K, int P) {
-		if (N < 2 || K < 1 || M < 1 || M > N || P < 1 || P > N) {
-			return 0;
-		}
-		int[][] dp = new int[K + 1][N + 1];
-		dp[0][P] = 1;
-		for (int i = 1; i <= K; i++) {
-			for (int j = 1; j <= N; j++) {
-				if (j == 1) {
-					dp[i][j] = dp[i - 1][2];
-				} else if (j == N) {
-					dp[i][j] = dp[i - 1][N - 1];
-				} else {
-					dp[i][j] = dp[i - 1][j - 1] + dp[i - 1][j + 1];
-				}
-			}
-		}
-		return dp[K][M];
-	}
-
-	public static int ways3(int N, int M, int K, int P) {
-		if (N < 2 || K < 1 || M < 1 || M > N || P < 1 || P > N) {
-			return 0;
-		}
-		int[] dp = new int[N + 1];
-		dp[P] = 1;
-		for (int i = 1; i <= K; i++) {
-			int leftUp = dp[1];
-			for (int j = 1; j <= N; j++) {
-				int tmp = dp[j];
-				if (j == 1) {
-					dp[j] = dp[j + 1];
-				} else if (j == N) {
-					dp[j] = leftUp;
-				} else {
-					dp[j] = leftUp + dp[j + 1];
-				}
-				leftUp = tmp;
-			}
-		}
-		return dp[M];
-	}
+			ans = cur == aim ? 1 : 0;
+		} else if (cur == 1) {
+			ans = process2(2, rest - 1, aim, N, dp);
+		} else if (cur == N) {
+			ans = process2(N - 1, rest - 1, aim, N, dp);
+		} else {
+			ans = process2(cur - 1, rest - 1, aim, N, dp) + process2(cur + 1, rest - 1, aim, N, dp);
+		}
+		dp[cur][rest] = ans;
+		return ans;
 
-	public static int ways4(int N, int M, int K, int P) {
-		if (N < 2 || K < 1 || M < 1 || M > N || P < 1 || P > N) {
-			return 0;
-		}
-		return process(N, 0, P, M, K);
 	}
 
-	public static int process(int N, int i, int j, int M, int K) {
-		if (i == K) {
-			return j == M ? 1 : 0;
-		}
-		if (j == 1) {
-			return process(N, i + 1, j + 1, M, K);
+	public static int ways3(int N, int start, int aim, int K) {
+		if (N < 2 || start < 1 || start > N || aim < 1 || aim > N || K < 1) {
+			return -1;
 		}
-		if (j == N) {
-			return process(N, i + 1, j - 1, M, K);
-		}
-		return process(N, i + 1, j + 1, M, K) + process(N, i + 1, j - 1, M, K);
-	}
-
-	public static int ways5(int N, int M, int K, int P) {
-		if (N < 2 || K < 1 || M < 1 || M > N || P < 1 || P > N) {
-			return 0;
-		}
-		int[][] dp = new int[K + 1][N + 1];
-		dp[K][M] = 1;
-		for (int i = K - 1; i >= 0; i--) {
-			for (int j = 1; j <= N; j++) {
-				if (j == 1) {
-					dp[i][j] = dp[i + 1][j + 1];
-				} else if (j == N) {
-					dp[i][j] = dp[i + 1][j - 1];
-				} else {
-					dp[i][j] = dp[i + 1][j + 1] + dp[i + 1][j - 1];
-				}
+		int[][] dp = new int[N + 1][K + 1];
+		dp[aim][0] = 1;
+		for (int rest = 1; rest <= K; rest++) {
+			dp[1][rest] = dp[2][rest - 1];
+			for (int cur = 2; cur < N; cur++) {
+				dp[cur][rest] = dp[cur - 1][rest - 1] + dp[cur + 1][rest - 1];
 			}
+			dp[N][rest] = dp[N - 1][rest - 1];
 		}
-		return dp[0][P];
+		return dp[start][K];
 	}
 
 	public static void main(String[] args) {
-		System.out.println(ways1(7, 4, 9, 5));
-		System.out.println(ways2(7, 4, 9, 5));
-		System.out.println(ways3(7, 4, 9, 5));
-		System.out.println(ways4(7, 4, 9, 5));
-		System.out.println(ways5(7, 4, 9, 5));
+		System.out.println(ways1(5, 2, 4, 6));
+		System.out.println(ways2(5, 2, 4, 6));
+		System.out.println(ways3(5, 2, 4, 6));
 	}
 
 }
diff --git a/src/class18/Code02_CardsInLine.java b/src/class18/Code02_CardsInLine.java
new file mode 100644
index 0000000..919a4cb
--- /dev/null
+++ b/src/class18/Code02_CardsInLine.java
@@ -0,0 +1,116 @@
+package class18;
+
+public class Code02_CardsInLine {
+
+	// 根据规则，返回获胜者的分数
+	public static int win1(int[] arr) {
+		if (arr == null || arr.length == 0) {
+			return 0;
+		}
+		int first = f1(arr, 0, arr.length - 1);
+		int second = g1(arr, 0, arr.length - 1);
+		return Math.max(first, second);
+	}
+
+	// arr[L..R]，先手获得的最好分数返回
+	public static int f1(int[] arr, int L, int R) {
+		if (L == R) {
+			return arr[L];
+		}
+		int p1 = arr[L] + g1(arr, L + 1, R);
+		int p2 = arr[R] + g1(arr, L, R - 1);
+		return Math.max(p1, p2);
+	}
+
+	// // arr[L..R]，后手获得的最好分数返回
+	public static int g1(int[] arr, int L, int R) {
+		if (L == R) {
+			return 0;
+		}
+		int p1 = f1(arr, L + 1, R); // 对手拿走了L位置的数
+		int p2 = f1(arr, L, R - 1); // 对手拿走了R位置的数
+		return Math.min(p1, p2);
+	}
+
+	public static int win2(int[] arr) {
+		if (arr == null || arr.length == 0) {
+			return 0;
+		}
+		int N = arr.length;
+		int[][] fmap = new int[N][N];
+		int[][] gmap = new int[N][N];
+		for (int i = 0; i < N; i++) {
+			for (int j = 0; j < N; j++) {
+				fmap[i][j] = -1;
+				gmap[i][j] = -1;
+			}
+		}
+		int first = f2(arr, 0, arr.length - 1, fmap, gmap);
+		int second = g2(arr, 0, arr.length - 1, fmap, gmap);
+		return Math.max(first, second);
+	}
+
+	// arr[L..R]，先手获得的最好分数返回
+	public static int f2(int[] arr, int L, int R, int[][] fmap, int[][] gmap) {
+		if (fmap[L][R] != -1) {
+			return fmap[L][R];
+		}
+		int ans = 0;
+		if (L == R) {
+			ans = arr[L];
+		} else {
+			int p1 = arr[L] + g2(arr, L + 1, R, fmap, gmap);
+			int p2 = arr[R] + g2(arr, L, R - 1, fmap, gmap);
+			ans = Math.max(p1, p2);
+		}
+		fmap[L][R] = ans;
+		return ans;
+	}
+
+	// // arr[L..R]，后手获得的最好分数返回
+	public static int g2(int[] arr, int L, int R, int[][] fmap, int[][] gmap) {
+		if (gmap[L][R] != -1) {
+			return gmap[L][R];
+		}
+		int ans = 0;
+		if (L != R) {
+			int p1 = f2(arr, L + 1, R, fmap, gmap); // 对手拿走了L位置的数
+			int p2 = f2(arr, L, R - 1, fmap, gmap); // 对手拿走了R位置的数
+			ans = Math.min(p1, p2);
+		}
+		gmap[L][R] = ans;
+		return ans;
+	}
+
+	public static int win3(int[] arr) {
+		if (arr == null || arr.length == 0) {
+			return 0;
+		}
+		int N = arr.length;
+		int[][] fmap = new int[N][N];
+		int[][] gmap = new int[N][N];
+		for (int i = 0; i < N; i++) {
+			fmap[i][i] = arr[i];
+		}
+		for (int startCol = 1; startCol < N; startCol++) {
+			int L = 0;
+			int R = startCol;
+			while (R < N) {
+				fmap[L][R] = Math.max(arr[L] + gmap[L + 1][R], arr[R] + gmap[L][R - 1]);
+				gmap[L][R] = Math.min(fmap[L + 1][R], fmap[L][R - 1]);
+				L++;
+				R++;
+			}
+		}
+		return Math.max(fmap[0][N - 1], gmap[0][N - 1]);
+	}
+
+	public static void main(String[] args) {
+		int[] arr = { 5, 7, 4, 5, 8, 1, 6, 0, 3, 4, 6, 1, 7 };
+		System.out.println(win1(arr));
+		System.out.println(win2(arr));
+		System.out.println(win3(arr));
+
+	}
+
+}
diff --git a/src/class18/Code03_CardsInLine.java b/src/class18/Code03_CardsInLine.java
deleted file mode 100644
index 4d74b32..0000000
--- a/src/class18/Code03_CardsInLine.java
+++ /dev/null
@@ -1,56 +0,0 @@
-package class18;
-
-public class Code03_CardsInLine {
-
-	public static int win1(int[] arr) {
-		if (arr == null || arr.length == 0) {
-			return 0;
-		}
-		return Math.max(f(arr, 0, arr.length - 1), s(arr, 0, arr.length - 1));
-	}
-
-	public static int f(int[] arr, int L, int R) {
-		if (L == R) {
-			return arr[L];
-		}
-		return Math.max(arr[L] + s(arr, L + 1, R), arr[R] + s(arr, L, R - 1));
-	}
-
-	public static int s(int[] arr, int L, int R) {
-		if (L == R) {
-			return 0;
-		}
-		return Math.min(f(arr, L + 1, R), f(arr, L, R - 1));
-	}
-
-	public static int windp(int[] arr) {
-		if (arr == null || arr.length == 0) {
-			return 0;
-		}
-		int N = arr.length;
-		int[][] f = new int[N][N];
-		int[][] s = new int[N][N];
-		for (int i = 0; i < N; i++) {
-			f[i][i] = arr[i];
-		}
-		for (int col = 1; col < N; col++) {
-			int L = 0;
-			int R = col;
-			while (L < N && R < N) {
-				f[L][R] = Math.max(arr[L] + s[L + 1][R], arr[R] + s[L][R - 1]);
-				s[L][R] = Math.min(f[L + 1][R], f[L][R - 1]);
-				L++;
-				R++;
-			}
-		}
-		return Math.max(f[0][N - 1], s[0][N - 1]);
-	}
-
-	public static void main(String[] args) {
-		int[] arr = { 5, 7, 4, 5, 8, 1, 6, 0, 3, 4, 6, 1, 7 };
-		System.out.println(win1(arr));
-		System.out.println(windp(arr));
-
-	}
-
-}
diff --git a/src/class18/Code02_Knapsack.java b/src/class18/Code03_Knapsack.java
similarity index 98%
rename from src/class18/Code02_Knapsack.java
rename to src/class18/Code03_Knapsack.java
index 0bcb2e0..d80c76e 100644
--- a/src/class18/Code02_Knapsack.java
+++ b/src/class18/Code03_Knapsack.java
@@ -1,6 +1,6 @@
 package class18;
 
-public class Code02_Knapsack {
+public class Code03_Knapsack {
 
 	public static int getMaxValue(int[] w, int[] v, int bag) {
 		return process(w, v, 0, 0, bag);
diff --git a/src/class18/Code05_StickersToSpellWord.java b/src/class18/Code05_StickersToSpellWord.java
index 5e574c8..fcb375c 100644
--- a/src/class18/Code05_StickersToSpellWord.java
+++ b/src/class18/Code05_StickersToSpellWord.java
@@ -72,7 +72,7 @@ public class Code05_StickersToSpellWord {
 		int min = Integer.MAX_VALUE;
 		for (int i = 0; i < N; i++) {
 			int[] sticker = stickers[i];
-			// 关键优化(重要的剪枝!这一步也是贪心!)
+			// 最关键的优化(重要的剪枝!这一步也是贪心!)
 			if (sticker[target[0] - 'a'] > 0) {
 				StringBuilder builder = new StringBuilder();
 				for (int j = 0; j < 26; j++) {