diff --git a/src/class18/Code03_Knapsack.java b/src/class18/Code03_Knapsack.java
deleted file mode 100644
index d80c76e..0000000
--- a/src/class18/Code03_Knapsack.java
+++ /dev/null
@@ -1,78 +0,0 @@
-package class18;
-
-public class Code03_Knapsack {
-
-	public static int getMaxValue(int[] w, int[] v, int bag) {
-		return process(w, v, 0, 0, bag);
-	}
-
-	// index... 最大价值
-	public static int process(int[] w, int[] v, int index, int alreadyW, int bag) {
-		if (alreadyW > bag) {
-			return -1;
-		}
-		// 重量没超
-		if (index == w.length) {
-			return 0;
-		}
-		int p1 = process(w, v, index + 1, alreadyW, bag);
-		int p2next = process(w, v, index + 1, alreadyW + w[index], bag);
-		int p2 = -1;
-		if (p2next != -1) {
-			p2 = v[index] + p2next;
-		}
-		return Math.max(p1, p2);
-
-	}
-
-	public static int maxValue(int[] w, int[] v, int bag) {
-		return process(w, v, 0, bag);
-	}
-
-	// 只剩下rest的空间了，
-	// index...货物自由选择，但是不要超过rest的空间
-	// 返回能够获得的最大价值
-	public static int process(int[] w, int[] v, int index, int rest) {
-		if (rest < 0) { // base case 1
-			return -1;
-		}
-		// rest >=0
-		if (index == w.length) { // base case 2
-			return 0;
-		}
-		// 有货也有空间
-		int p1 = process(w, v, index + 1, rest);
-		int p2 = -1;
-		int p2Next = process(w, v, index + 1, rest - w[index]);
-		if (p2Next != -1) {
-			p2 = v[index] + p2Next;
-		}
-		return Math.max(p1, p2);
-	}
-
-	public static int dpWay(int[] w, int[] v, int bag) {
-		int N = w.length;
-		int[][] dp = new int[N + 1][bag + 1];
-		// dp[N][...] = 0
-		for (int index = N - 1; index >= 0; index--) {
-			for (int rest = 0; rest <= bag; rest++) { // rest < 0
-				int p1 = dp[index + 1][rest];
-				int p2 = -1;
-				if (rest - w[index] >= 0) {
-					p2 = v[index] + dp[index + 1][rest - w[index]];
-				}
-				dp[index][rest] = Math.max(p1, p2);
-			}
-		}
-		return dp[0][bag];
-	}
-
-	public static void main(String[] args) {
-		int[] weights = { 3, 2, 4, 7 };
-		int[] values = { 5, 6, 3, 19 };
-		int bag = 11;
-		System.out.println(maxValue(weights, values, bag));
-		System.out.println(dpWay(weights, values, bag));
-	}
-
-}
diff --git a/src/class18/Code04_ConvertToLetterString.java b/src/class18/Code04_ConvertToLetterString.java
deleted file mode 100644
index 99e9825..0000000
--- a/src/class18/Code04_ConvertToLetterString.java
+++ /dev/null
@@ -1,69 +0,0 @@
-package class18;
-
-public class Code04_ConvertToLetterString {
-
-	public static int number(String str) {
-		if (str == null || str.length() == 0) {
-			return 0;
-		}
-		return process(str.toCharArray(), 0);
-	}
-
-	public static int process(char[] str, int i) {
-		if (i == str.length) {
-			return 1;
-		}
-		if (str[i] == '0') {
-			return 0;
-		}
-		if (str[i] == '1') {
-			int res = process(str, i + 1);
-			if (i + 1 < str.length) {
-				res += process(str, i + 2);
-			}
-			return res;
-		}
-		if (str[i] == '2') {
-			int res = process(str, i + 1);
-			if (i + 1 < str.length && (str[i + 1] >= '0' && str[i + 1] <= '6')) {
-				res += process(str, i + 2); // (i和i+1)作为单独的部分，后续有多少种方法
-			}
-			return res;
-		}
-		return process(str, i + 1);
-	}
-
-	public static int dp(String s) {
-		if (s == null || s.length() == 0) {
-			return 0;
-		}
-		char[] str = s.toCharArray();
-		int N = str.length;
-		int[] dp = new int[N + 1];
-		dp[N] = 1;
-		for (int i = N - 1; i >= 0; i--) {
-			if (str[i] == '0') {
-				dp[i] = 0;
-			} else if (str[i] == '1') {
-				dp[i] = dp[i + 1];
-				if (i + 1 < N) {
-					dp[i] += dp[i + 2];
-				}
-			} else if (str[i] == '2') {
-				dp[i] = dp[i + 1];
-				if (i + 1 < str.length && (str[i + 1] >= '0' && str[i + 1] <= '6')) {
-					dp[i] += dp[i + 2];
-				}
-			} else {
-				dp[i] = dp[i + 1];
-			}
-		}
-		return dp[0];
-	}
-
-	public static void main(String[] args) {
-		System.out.println(number("2132082"));
-		System.out.println(dp("2132082"));
-	}
-
-}
diff --git a/src/class19/Code01_Knapsack.java b/src/class19/Code01_Knapsack.java
new file mode 100644
index 0000000..ab355a2
--- /dev/null
+++ b/src/class19/Code01_Knapsack.java
@@ -0,0 +1,63 @@
+package class19;
+
+public class Code01_Knapsack {
+
+	// 所有的货，重量和价值，都在w和v数组里
+	// 为了方便，其中没有负数
+	// bag背包容量，不能超过这个载重
+	// 返回：不超重的情况下，能够得到的最大价值
+	public static int maxValue(int[] w, int[] v, int bag) {
+		if (w == null || v == null || w.length != v.length || w.length == 0) {
+			return 0;
+		}
+		// 尝试函数！
+		return process(w, v, 0, bag);
+	}
+
+	// index 0~N
+	// rest 负~bag
+	public static int process(int[] w, int[] v, int index, int rest) {
+		if (rest < 0) {
+			return -1;
+		}
+		if (index == w.length) {
+			return 0;
+		}
+		int p1 = process(w, v, index + 1, rest);
+		int p2 = 0;
+		int next = process(w, v, index + 1, rest - w[index]);
+		if (next != -1) {
+			p2 = v[index] + next;
+		}
+		return Math.max(p1, p2);
+	}
+
+	public static int dp(int[] w, int[] v, int bag) {
+		if (w == null || v == null || w.length != v.length || w.length == 0) {
+			return 0;
+		}
+		int N = w.length;
+		int[][] dp = new int[N + 1][bag + 1];
+		for (int index = N - 1; index >= 0; index--) {
+			for (int rest = 0; rest <= bag; rest++) {
+				int p1 = dp[index + 1][rest];
+				int p2 = 0;
+				int next = rest - w[index] < 0 ? -1 : dp[index + 1][rest - w[index]];
+				if (next != -1) {
+					p2 = v[index] + next;
+				}
+				dp[index][rest] = Math.max(p1, p2);
+			}
+		}
+		return dp[0][bag];
+	}
+
+	public static void main(String[] args) {
+		int[] weights = { 3, 2, 4, 7, 3, 1, 7 };
+		int[] values = { 5, 6, 3, 19, 12, 4, 2 };
+		int bag = 15;
+		System.out.println(maxValue(weights, values, bag));
+		System.out.println(dp(weights, values, bag));
+	}
+
+}
diff --git a/src/class19/Code01_LongestCommonSubsequence.java b/src/class19/Code01_LongestCommonSubsequence.java
deleted file mode 100644
index 36e94e4..0000000
--- a/src/class19/Code01_LongestCommonSubsequence.java
+++ /dev/null
@@ -1,58 +0,0 @@
-package class19;
-
-// 这个问题leetcode上可以直接测
-// 链接：https://leetcode.com/problems/longest-common-subsequence/
-public class Code01_LongestCommonSubsequence {
-
-	public static int longestCommonSubsequence1(String text1, String text2) {
-		if (text1 == null || text2 == null || text1.length() == 0 || text2.length() == 0) {
-			return 0;
-		}
-		return process(text1.toCharArray(), text2.toCharArray(), text1.length() - 1, text2.length() - 1);
-	}
-
-	// str1[0..i]与str2[0..j]最长公共子序列多长
-	public static int process(char[] str1, char[] str2, int i, int j) {
-		if (i == 0 && j == 0) {
-			return str1[0] == str2[0] ? 1 : 0;
-		}
-		if (i == 0) {
-			return str1[0] == str2[j] ? 1 : process(str1, str2, 0, j - 1);
-		}
-		if (j == 0) {
-			return str1[i] == str2[0] ? 1 : process(str1, str2, i - 1, 0);
-		}
-		int p1 = process(str1, str2, i - 1, j);
-		int p2 = process(str1, str2, i, j - 1);
-		int p3 = str1[i] == str2[j] ? (process(str1, str2, i - 1, j - 1) + 1) : 0;
-		return Math.max(Math.max(p1, p2), p3);
-	}
-
-	public static int longestCommonSubsequence2(String text1, String text2) {
-		if (text1 == null || text2 == null || text1.length() == 0 || text2.length() == 0) {
-			return 0;
-		}
-		char[] str1 = text1.toCharArray();
-		char[] str2 = text2.toCharArray();
-		int N = str1.length;
-		int M = str2.length;
-		int[][] dp = new int[N][M];
-		dp[0][0] = str1[0] == str2[0] ? 1 : 0;
-		for (int i = 1; i < N; i++) {
-			dp[i][0] = str1[i] == str2[0] ? 1 : dp[i - 1][0];
-		}
-		for (int j = 1; j < M; j++) {
-			dp[0][j] = str1[0] == str2[j] ? 1 : dp[0][j - 1];
-		}
-		for (int i = 1; i < N; i++) {
-			for (int j = 1; j < M; j++) {
-				dp[i][j] = Math.max(dp[i - 1][j], dp[i][j - 1]);
-				if (str1[i] == str2[j]) {
-					dp[i][j] = Math.max(dp[i][j], dp[i - 1][j - 1] + 1);
-				}
-			}
-		}
-		return dp[N - 1][M - 1];
-	}
-
-}
diff --git a/src/class19/Code02_ConvertToLetterString.java b/src/class19/Code02_ConvertToLetterString.java
new file mode 100644
index 0000000..6b14299
--- /dev/null
+++ b/src/class19/Code02_ConvertToLetterString.java
@@ -0,0 +1,58 @@
+package class19;
+
+public class Code02_ConvertToLetterString {
+
+	// str只含有数字字符0~9
+	// 返回多少种转化方案
+	public static int number(String str) {
+		if (str == null || str.length() == 0) {
+			return 0;
+		}
+		return process(str.toCharArray(), 0);
+	}
+
+	// str[0..i-1]转化无需过问
+	// str[i.....]去转化，返回有多少种转化方法
+	public static int process(char[] str, int i) {
+		if (i == str.length) {
+			return 1;
+		}
+		// i没到最后，说明有字符
+		if (str[i] == '0') { // 之前的决定有问题
+			return 0;
+		}
+		// str[i] != '0'
+		// 可能性一，i单转
+		int ways = process(str, i + 1);
+		if (i + 1 < str.length && (str[i] - '0') * 10 + str[i + 1] - '0' < 27) {
+			ways += process(str, i + 2);
+		}
+		return ways;
+	}
+
+	public static int dp(String s) {
+		if (s == null || s.length() == 0) {
+			return 0;
+		}
+		char[] str = s.toCharArray();
+		int N = str.length;
+		int[] dp = new int[N + 1];
+		dp[N] = 1;
+		for (int i = N - 1; i >= 0; i--) {
+			if (str[i] != '0') {
+				int ways = dp[i + 1];
+				if (i + 1 < str.length && (str[i] - '0') * 10 + str[i + 1] - '0' < 27) {
+					ways += dp[i + 2];
+				}
+				dp[i] = ways;
+			}
+		}
+		return dp[0];
+	}
+
+	public static void main(String[] args) {
+		System.out.println(number("7210231231232031203123"));
+		System.out.println(dp("7210231231232031203123"));
+	}
+
+}
diff --git a/src/class18/Code05_StickersToSpellWord.java b/src/class19/Code03_StickersToSpellWord.java
similarity index 89%
rename from src/class18/Code05_StickersToSpellWord.java
rename to src/class19/Code03_StickersToSpellWord.java
index fcb375c..90b3a4d 100644
--- a/src/class18/Code05_StickersToSpellWord.java
+++ b/src/class19/Code03_StickersToSpellWord.java
@@ -1,15 +1,18 @@
-package class18;
+package class19;
 
 import java.util.HashMap;
 
 // 本题测试链接：https://leetcode.com/problems/stickers-to-spell-word
-public class Code05_StickersToSpellWord {
+public class Code03_StickersToSpellWord {
 
 	public static int minStickers1(String[] stickers, String target) {
 		int ans = process1(stickers, target);
 		return ans == Integer.MAX_VALUE ? -1 : ans;
 	}
 
+	// 所有贴纸stickers，每一种贴纸都有无穷张
+	// target
+	// 最少张数
 	public static int process1(String[] stickers, String target) {
 		if (target.length() == 0) {
 			return 0;
@@ -59,10 +62,17 @@ public class Code05_StickersToSpellWord {
 		return ans == Integer.MAX_VALUE ? -1 : ans;
 	}
 
+	// stickers[i] 数组，当初i号贴纸的字符统计 int[][] stickers -> 所有的贴纸
+	// 每一种贴纸都有无穷张
+	// 返回搞定target的最少张数
+	// 最少张数
 	public static int process2(int[][] stickers, String t) {
 		if (t.length() == 0) {
 			return 0;
 		}
+		// target做出词频统计
+		// target  aabbc  2 2 1..
+		//                0 1 2..
 		char[] target = t.toCharArray();
 		int[] tcounts = new int[26];
 		for (char cha : target) {
@@ -71,6 +81,7 @@ public class Code05_StickersToSpellWord {
 		int N = stickers.length;
 		int min = Integer.MAX_VALUE;
 		for (int i = 0; i < N; i++) {
+			// 尝试第一张贴纸是谁
 			int[] sticker = stickers[i];
 			// 最关键的优化(重要的剪枝!这一步也是贪心!)
 			if (sticker[target[0] - 'a'] > 0) {
diff --git a/src/class19/Code04_LongestCommonSubsequence.java b/src/class19/Code04_LongestCommonSubsequence.java
new file mode 100644
index 0000000..78862f9
--- /dev/null
+++ b/src/class19/Code04_LongestCommonSubsequence.java
@@ -0,0 +1,67 @@
+package class19;
+
+// 这个问题leetcode上可以直接测
+// 链接：https://leetcode.com/problems/longest-common-subsequence/
+public class Code04_LongestCommonSubsequence {
+
+	public static int longestCommonSubsequence1(String s1, String s2) {
+		if (s1 == null || s2 == null || s1.length() == 0 || s2.length() == 0) {
+			return 0;
+		}
+		char[] str1 = s1.toCharArray();
+		char[] str2 = s2.toCharArray();
+		// 尝试
+		return process1(str1, str2, str1.length - 1, str2.length - 1);
+	}
+
+	public static int process1(char[] str1, char[] str2, int i, int j) {
+		if (i == 0 && j == 0) {
+			return str1[i] == str2[j] ? 1 : 0;
+		} else if (i == 0) {
+			if (str1[i] == str2[j]) {
+				return 1;
+			} else {
+				return process1(str1, str2, i, j - 1);
+			}
+		} else if (j == 0) {
+			if (str1[i] == str2[j]) {
+				return 1;
+			} else {
+				return process1(str1, str2, i - 1, j);
+			}
+		} else { // i != 0 && j != 0
+			int p1 = process1(str1, str2, i - 1, j);
+			int p2 = process1(str1, str2, i, j - 1);
+			int p3 = str1[i] == str2[j] ? (1 + process1(str1, str2, i - 1, j - 1)) : 0;
+			return Math.max(p1, Math.max(p2, p3));
+		}
+	}
+
+	public static int longestCommonSubsequence2(String s1, String s2) {
+		if (s1 == null || s2 == null || s1.length() == 0 || s2.length() == 0) {
+			return 0;
+		}
+		char[] str1 = s1.toCharArray();
+		char[] str2 = s2.toCharArray();
+		int N = str1.length;
+		int M = str2.length;
+		int[][] dp = new int[N][M];
+		dp[0][0] = str1[0] == str2[0] ? 1 : 0;
+		for (int j = 1; j < M; j++) {
+			dp[0][j] = str1[0] == str2[j] ? 1 : dp[0][j - 1];
+		}
+		for (int i = 1; i < N; i++) {
+			dp[i][0] = str1[i] == str2[0] ? 1 : dp[i - 1][0];
+		}
+		for (int i = 1; i < N; i++) {
+			for (int j = 1; j < M; j++) {
+				int p1 = dp[i - 1][j];
+				int p2 = dp[i][j - 1];
+				int p3 = str1[i] == str2[j] ? (1 + dp[i - 1][j - 1]) : 0;
+				dp[i][j] = Math.max(p1, Math.max(p2, p3));
+			}
+		}
+		return dp[N - 1][M - 1];
+	}
+
+}
diff --git a/src/class19/Code02_PalindromeSubsequence.java b/src/class20/Code01_PalindromeSubsequence.java
similarity index 96%
rename from src/class19/Code02_PalindromeSubsequence.java
rename to src/class20/Code01_PalindromeSubsequence.java
index 94cf109..062f242 100644
--- a/src/class19/Code02_PalindromeSubsequence.java
+++ b/src/class20/Code01_PalindromeSubsequence.java
@@ -1,7 +1,7 @@
-package class19;
+package class20;
 
 // 测试链接：https://leetcode.com/problems/longest-palindromic-subsequence/
-public class Code02_PalindromeSubsequence {
+public class Code01_PalindromeSubsequence {
 
 	public static int longestPalindromeSubseq1(String s) {
 		if (s == null || s.length() == 0) {
diff --git a/src/class19/Code03_HorseJump.java b/src/class20/Code02_HorseJump.java
similarity index 97%
rename from src/class19/Code03_HorseJump.java
rename to src/class20/Code02_HorseJump.java
index a560edd..4744f1e 100644
--- a/src/class19/Code03_HorseJump.java
+++ b/src/class20/Code02_HorseJump.java
@@ -1,6 +1,6 @@
-package class19;
+package class20;
 
-public class Code03_HorseJump {
+public class Code02_HorseJump {
 
 	public static int ways(int a, int b, int step) {
 		return f(0, 0, step, a, b);
diff --git a/src/class19/Code04_Coffee.java b/src/class20/Code03_Coffee.java
similarity index 99%
rename from src/class19/Code04_Coffee.java
rename to src/class20/Code03_Coffee.java
index fec9a7b..070faa9 100644
--- a/src/class19/Code04_Coffee.java
+++ b/src/class20/Code03_Coffee.java
@@ -1,4 +1,4 @@
-package class19;
+package class20;
 
 import java.util.Arrays;
 import java.util.Comparator;
@@ -12,7 +12,7 @@ import java.util.PriorityQueue;
 // 洗杯子的机器洗完一个杯子时间为a，任何一个杯子自然挥发干净的时间为b。
 // 四个参数：arr, n, a, b
 // 假设时间点从0开始，返回所有人喝完咖啡并洗完咖啡杯的全部过程结束后，至少来到什么时间点。
-public class Code04_Coffee {
+public class Code03_Coffee {
 
 	// 方法一：暴力尝试方法
 	public static int minTime1(int[] arr, int n, int a, int b) {
diff --git a/src/class19/Code05_MinPathSum.java b/src/class20/Code04_MinPathSum.java
similarity index 97%
rename from src/class19/Code05_MinPathSum.java
rename to src/class20/Code04_MinPathSum.java
index bc8efeb..80d3e0b 100644
--- a/src/class19/Code05_MinPathSum.java
+++ b/src/class20/Code04_MinPathSum.java
@@ -1,6 +1,6 @@
-package class19;
+package class20;
 
-public class Code05_MinPathSum {
+public class Code04_MinPathSum {
 
 	public static int minPathSum1(int[][] m) {
 		if (m == null || m.length == 0 || m[0] == null || m[0].length == 0) {
diff --git a/src/class20/Code01_CoinsWayEveryPaperDifferent.java b/src/class21/Code01_CoinsWayEveryPaperDifferent.java
similarity index 99%
rename from src/class20/Code01_CoinsWayEveryPaperDifferent.java
rename to src/class21/Code01_CoinsWayEveryPaperDifferent.java
index 008910d..7df1873 100644
--- a/src/class20/Code01_CoinsWayEveryPaperDifferent.java
+++ b/src/class21/Code01_CoinsWayEveryPaperDifferent.java
@@ -1,4 +1,4 @@
-package class20;
+package class21;
 
 public class Code01_CoinsWayEveryPaperDifferent {
 
diff --git a/src/class20/Code02_CoinsWayNoLimit.java b/src/class21/Code02_CoinsWayNoLimit.java
similarity index 99%
rename from src/class20/Code02_CoinsWayNoLimit.java
rename to src/class21/Code02_CoinsWayNoLimit.java
index 95dd573..0b3a5a0 100644
--- a/src/class20/Code02_CoinsWayNoLimit.java
+++ b/src/class21/Code02_CoinsWayNoLimit.java
@@ -1,4 +1,4 @@
-package class20;
+package class21;
 
 public class Code02_CoinsWayNoLimit {
 
diff --git a/src/class20/Code03_CoinsWaySameValueSamePapper.java b/src/class21/Code03_CoinsWaySameValueSamePapper.java
similarity index 99%
rename from src/class20/Code03_CoinsWaySameValueSamePapper.java
rename to src/class21/Code03_CoinsWaySameValueSamePapper.java
index 357fb37..fddbe7e 100644
--- a/src/class20/Code03_CoinsWaySameValueSamePapper.java
+++ b/src/class21/Code03_CoinsWaySameValueSamePapper.java
@@ -1,4 +1,4 @@
-package class20;
+package class21;
 
 import java.util.HashMap;
 import java.util.Map.Entry;
diff --git a/src/class20/Code04_MinCoinsOnePaper.java b/src/class21/Code04_MinCoinsOnePaper.java
similarity index 99%
rename from src/class20/Code04_MinCoinsOnePaper.java
rename to src/class21/Code04_MinCoinsOnePaper.java
index 02e48a1..7ad8a7e 100644
--- a/src/class20/Code04_MinCoinsOnePaper.java
+++ b/src/class21/Code04_MinCoinsOnePaper.java
@@ -1,4 +1,4 @@
-package class20;
+package class21;
 
 import java.util.HashMap;
 import java.util.Map.Entry;
diff --git a/src/class20/Code05_MinCoinsNoLimit.java b/src/class21/Code05_MinCoinsNoLimit.java
similarity index 99%
rename from src/class20/Code05_MinCoinsNoLimit.java
rename to src/class21/Code05_MinCoinsNoLimit.java
index b41ad45..31e7c3d 100644
--- a/src/class20/Code05_MinCoinsNoLimit.java
+++ b/src/class21/Code05_MinCoinsNoLimit.java
@@ -1,4 +1,4 @@
-package class20;
+package class21;
 
 public class Code05_MinCoinsNoLimit {
 
diff --git a/src/class21/Code01_BobDie.java b/src/class22/Code01_BobDie.java
similarity index 98%
rename from src/class21/Code01_BobDie.java
rename to src/class22/Code01_BobDie.java
index 211be20..ba68e3a 100644
--- a/src/class21/Code01_BobDie.java
+++ b/src/class22/Code01_BobDie.java
@@ -1,4 +1,4 @@
-package class21;
+package class22;
 
 public class Code01_BobDie {
 
diff --git a/src/class21/Code02_KillMonster.java b/src/class22/Code02_KillMonster.java
similarity index 99%
rename from src/class21/Code02_KillMonster.java
rename to src/class22/Code02_KillMonster.java
index fbc2871..3465ab2 100644
--- a/src/class21/Code02_KillMonster.java
+++ b/src/class22/Code02_KillMonster.java
@@ -1,4 +1,4 @@
-package class21;
+package class22;
 
 public class Code02_KillMonster {
 
diff --git a/src/class21/Code03_SplitSumClosed.java b/src/class22/Code03_SplitSumClosed.java
similarity index 99%
rename from src/class21/Code03_SplitSumClosed.java
rename to src/class22/Code03_SplitSumClosed.java
index c9bb573..85ce147 100644
--- a/src/class21/Code03_SplitSumClosed.java
+++ b/src/class22/Code03_SplitSumClosed.java
@@ -1,4 +1,4 @@
-package class21;
+package class22;
 
 import java.util.TreeSet;
 
diff --git a/src/class21/Code04_SplitSumClosedSizeHalf.java b/src/class22/Code04_SplitSumClosedSizeHalf.java
similarity index 99%
rename from src/class21/Code04_SplitSumClosedSizeHalf.java
rename to src/class22/Code04_SplitSumClosedSizeHalf.java
index bdfffff..42d4d79 100644
--- a/src/class21/Code04_SplitSumClosedSizeHalf.java
+++ b/src/class22/Code04_SplitSumClosedSizeHalf.java
@@ -1,4 +1,4 @@
-package class21;
+package class22;
 
 import java.util.TreeSet;
 
diff --git a/src/class21/Code05_NQueens.java b/src/class22/Code05_NQueens.java
similarity index 99%
rename from src/class21/Code05_NQueens.java
rename to src/class22/Code05_NQueens.java
index de49c7a..5bbe7f1 100644
--- a/src/class21/Code05_NQueens.java
+++ b/src/class22/Code05_NQueens.java
@@ -1,4 +1,4 @@
-package class21;
+package class22;
 
 public class Code05_NQueens {