diff --git a/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode17.java b/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode17.java
index 29cb0f7..8650adc 100644
--- a/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode17.java
+++ b/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode17.java
@@ -15,15 +15,14 @@ public class LeetCode17 {
 
     @Test
     public void test() {
-        String digits = "";
-        new Solution().letterCombinations(digits).forEach(s -> System.out.println(s));
+        String digits = "23";
+        new Solution1().letterCombinations(digits).forEach(s -> System.out.println(s));
     }
 
     class Solution {
         List<String> map = new ArrayList<>();
-
         List<String> result = new ArrayList<>();
-
+        // 记录 路径
         StringBuilder s = new StringBuilder();
 
         public List<String> letterCombinations(String digits) {
@@ -59,6 +58,7 @@ public class LeetCode17 {
             int c = digits.charAt(number) - 48;
             String sValue = map.get(c);
             int length = sValue.length();
+            // 对每个字符处理
             for (int i = 0; i < length; i++) {
                 char c1 = sValue.charAt(i);
                 s.append(c1);
@@ -67,4 +67,53 @@ public class LeetCode17 {
             }
         }
     }
+
+    class Solution1 {
+        List<String> map = new ArrayList<>();
+        List<String> result = new ArrayList<>();
+        // 记录 路径
+        StringBuilder s = new StringBuilder();
+
+        public List<String> letterCombinations(String digits) {
+            if (digits.length() == 0) {
+                return result;
+            }
+            map.add(0, null);
+            map.add(1, null);
+            map.add(2, "abc");
+            map.add(3, "def");
+            map.add(4, "ghi");
+            map.add(5, "jkl");
+            map.add(6, "mno");
+            map.add(7, "pqrs");
+            map.add(8, "tuv");
+            map.add(9, "wxyz");
+
+            backTracking(digits, 0);
+            return result;
+        }
+
+        /**
+         *
+         * @param digits 原始字符 23
+         * @param number 使用到第几个字符
+         */
+        void backTracking(String digits, int number) {
+            if (s.length() == digits.length()) {
+                result.add(new String(s.toString()));
+                return;
+            }
+
+            int c = digits.charAt(number) - 48;
+            String sValue = map.get(c);
+            int length = sValue.length();
+            // 对每个字符处理
+            for (int i = 0; i < length; i++) {
+                char c1 = sValue.charAt(i);
+                s.append(c1);
+                backTracking(digits, number + 1);
+                s.deleteCharAt(s.length() - 1);
+            }
+        }
+    }
 }
diff --git a/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode39.java b/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode39.java
new file mode 100644
index 0000000..ba469d2
--- /dev/null
+++ b/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode39.java
@@ -0,0 +1,51 @@
+package cn.whaifree.leetCode.BackTracking;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/2/6 23:57
+ * @注释
+ */
+public class LeetCode39 {
+
+    @Test
+    public void test() {
+        new Solution().combinationSum(new int[]{2, 3, 6, 7}, 7).forEach(
+                list -> {
+                    list.forEach(System.out::print);
+                    System.out.println();
+                }
+        );
+    }
+
+
+    class Solution {
+
+        List<List<Integer>> res = new ArrayList<>();
+        List<Integer> path = new ArrayList<>();
+        public List<List<Integer>> combinationSum(int[] candidates, int target) {
+            backTracking(candidates, 0, target);
+            return res;
+        }
+
+        public void backTracking(int[] candidates, int index, int need) {
+            if (need < 0) {
+                return;
+            }
+            if (need == 0) {
+                res.add(new ArrayList<>(path));
+            }
+            for (int i = index; i < candidates.length ; i++) {
+                path.add(candidates[i]);
+                backTracking(candidates, i, need - candidates[i]);
+                path.remove(path.size() - 1);
+            }
+        }
+
+    }
+}
diff --git a/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode40.java b/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode40.java
new file mode 100644
index 0000000..a040d47
--- /dev/null
+++ b/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode40.java
@@ -0,0 +1,76 @@
+package cn.whaifree.leetCode.BackTracking;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.LinkedList;
+import java.util.List;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/2/6 23:57
+ * @注释
+ */
+public class LeetCode40 {
+
+    @Test
+    public void test() {
+        new Solution().combinationSum2(new int[]{10,1,2,7,6,1,5}, 8).forEach(
+                list -> {
+                    list.forEach(System.out::print);
+                    System.out.println();
+                }
+        );
+    }
+
+
+    class Solution {
+
+        List<List<Integer>> res = new LinkedList<>();
+        List<Integer> path = new ArrayList<>();
+        boolean[] used;
+
+        /**
+         * 结果不能重复！！！！
+         * 1. 排序candidates
+         * 2. 使用used[]数组记录使用
+         * 3. 一旦candi[index]==cnadi[index-1]&&user[index]=true，那么就是用过的，不添加
+         * @param candidates
+         * @param target
+         * @return
+         */
+        public List<List<Integer>> combinationSum2(int[] candidates, int target) {
+            Arrays.sort(candidates);
+            used = new boolean[candidates.length];
+            backTracking(candidates, 0, target);
+            return res;
+        }
+
+        public void backTracking(int[] candidates, int index, int need) {
+            if (need < 0) {
+                return;
+            }
+            if (need == 0) {
+                res.add(new ArrayList<>(path));
+            }
+            for (int i = index; i < candidates.length ; i++) {
+
+                // 取第一个数先 used放1，再放0；
+                // 取第二个数时，如果can两个数相同，并且used为0，表示前一个数用过。
+                // 如果 used为1，表示还没从递归中出来，即是【1，1】中的第二个1，是需要的
+                if (i > 0 && candidates[i] == candidates[i - 1] && used[i - 1] == false) {
+                    continue;
+                }
+
+                path.add(candidates[i]);
+                used[i] = true;
+                backTracking(candidates, i+1, need - candidates[i]);
+                used[i] = false;
+                path.remove(path.size() - 1);
+            }
+        }
+
+    }
+}
diff --git a/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode110.java b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode110.java
new file mode 100644
index 0000000..37c1157
--- /dev/null
+++ b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode110.java
@@ -0,0 +1,47 @@
+package cn.whaifree.redo.redo_24_2_4;
+
+import cn.whaifree.leetCode.model.TreeNode;
+import org.junit.Test;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/2/3 19:17
+ * @注释
+ */
+public class LeetCode110 {
+
+    @Test
+    public void test() {
+        System.out.println(new Solution().isBalanced(TreeNode.constructTreeByArray(1,null,2,null,3)));
+    }
+    class Solution {
+        public boolean isBalanced(TreeNode root) {
+            return balance(root) != -1;
+        }
+
+        public int balance(TreeNode root) {
+
+            if (root == null) {
+                return 0;
+            }
+            int right = balance(root.right);
+            if (right == -1) {
+                return -1;
+            }
+            int left = balance(root.left);
+            if (left == -1) {
+                return -1;
+            }
+
+            // 左右两边高度差
+            if (Math.abs(right - left) > 1) {
+                return -1;
+            }
+
+            // 返回高度
+            return Math.max(right, left)+ 1;
+        }
+
+    }
+}
diff --git a/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode203.java b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode203.java
new file mode 100644
index 0000000..c5ae95b
--- /dev/null
+++ b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode203.java
@@ -0,0 +1,34 @@
+package cn.whaifree.redo.redo_24_2_4;
+
+import cn.whaifree.leetCode.model.ListNode;
+import org.junit.Test;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/2/6 23:05
+ * @注释
+ */
+public class LeetCode203 {
+
+    @Test
+    public void test() {
+        new Solution().removeElements(ListNode.listNodeFromArray(new int[]{1, 2, 3, 4}), 4).printList();
+    }
+
+
+    class Solution {
+        public ListNode removeElements(ListNode head, int val) {
+            if (head == null) {
+                return null;
+            }
+
+            head.next = removeElements(head.next, val);
+            if (head.val == val) {
+                return head.next;
+            }else {
+                return head;
+            }
+        }
+    }
+}
diff --git a/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode206.java b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode206.java
new file mode 100644
index 0000000..58cab4f
--- /dev/null
+++ b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode206.java
@@ -0,0 +1,39 @@
+package cn.whaifree.redo.redo_24_2_4;
+
+import cn.whaifree.leetCode.model.ListNode;
+import org.junit.Test;
+
+import java.util.List;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/2/6 23:18
+ * @注释
+ */
+public class LeetCode206 {
+
+    @Test
+    public void test() {
+        new Solution().reverseList(ListNode.listNodeFromArray(new int[]{1, 2, 3})).printList();
+    }
+
+    class Solution {
+        public ListNode reverseList(ListNode head) {
+
+            if (head == null) {
+                return null;
+            }
+            return reverse(null, head);
+        }
+
+        public ListNode reverse(ListNode pre, ListNode after) {
+            if (after == null) {
+                return pre;
+            }
+            ListNode afterNext = after.next;
+            after.next = pre;
+            return reverse(after, afterNext);
+        }
+    }
+}
diff --git a/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode216.java b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode216.java
new file mode 100644
index 0000000..18ecbcf
--- /dev/null
+++ b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode216.java
@@ -0,0 +1,62 @@
+package cn.whaifree.redo.redo_24_2_4;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/2/4 13:07
+ * @注释
+ */
+public class LeetCode216 {
+
+    @Test
+    public void test() {
+        for (List<Integer> integers : new Solution().combinationSum3(9, 45)) {
+            System.out.println(integers);
+        }
+    }
+
+
+
+    class Solution {
+
+        List<List<Integer>> res = new ArrayList<>();
+        List<Integer> path = new ArrayList<>();
+        /**
+         * 找出所有相加之和为 n 的 k 个数的组合，且满足下列条件：
+         * @param k
+         * @param n
+         * @return
+         */
+        public List<List<Integer>> combinationSum3(int k, int n) {
+            recursion(1, 9, k, n);
+            return res;
+        }
+
+        void recursion(int start, int end, int k, int needNumber) {
+
+            if (needNumber == 0 && path.size() == k) {
+                res.add(new ArrayList<>(path));
+                return;
+            }
+
+            for (int i = start; i <= end; i++) {
+                // 标准剪枝
+                if (end - i + 1 < k - path.size()) {
+                    return;
+                }
+                path.add(i);
+                recursion(i + 1, end, k, needNumber - i);
+                path.remove(path.size() - 1);
+                // 如果 新加的数i 比 需要的数needNumber大，那么加这些正数是多余的操作
+                if (needNumber <= i) {
+                    return;
+                }
+            }
+        }
+    }
+}
diff --git a/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode222.java b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode222.java
new file mode 100644
index 0000000..080650a
--- /dev/null
+++ b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode222.java
@@ -0,0 +1,52 @@
+package cn.whaifree.redo.redo_24_2_4;
+
+import cn.whaifree.leetCode.model.TreeNode;
+import org.junit.Test;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/2/3 20:04
+ * @注释
+ */
+public class LeetCode222 {
+
+    @Test
+    public void test() {
+        TreeNode treeNode = TreeNode.constructTree(new Integer[]{1, 2, 3, 4, 5, 6});
+        treeNode.printTree();
+        System.out.println(new Solution().countNodes(treeNode));
+    }
+
+   class Solution{
+       /**
+        * 完全二叉树二分查找
+        * @param root
+        * @return
+        */
+       public int countNodes(TreeNode root) {
+           if (root==null) return 0;
+
+           TreeNode left = root.left;
+           int leftDepth = 0;
+           while (left != null) {
+               leftDepth++;
+               left = left.left;
+           }
+
+           TreeNode right = root.right;
+           int rightDepth = 0;
+           while (right != null) {
+               rightDepth++;
+               right = right.right;
+           }
+
+           if (leftDepth == rightDepth) {
+               return (2 << leftDepth) - 1;
+           }
+           return countNodes(root.left) + countNodes(root.right) + 1;
+       }
+   }
+
+
+}
diff --git a/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode236.java b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode236.java
new file mode 100644
index 0000000..e92187c
--- /dev/null
+++ b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode236.java
@@ -0,0 +1,44 @@
+package cn.whaifree.redo.redo_24_2_4;
+
+import cn.whaifree.leetCode.model.TreeNode;
+import org.junit.Test;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/2/4 0:08
+ * @注释
+ */
+public class LeetCode236 {
+
+    @Test
+    public void test() {
+        TreeNode treeNode = TreeNode.constructTreeByArray(1, 2, 3, 4, 5, 6);
+        new Solution().lowestCommonAncestor(treeNode, treeNode.right, treeNode.left).printTree();
+    }
+
+    class Solution {
+        public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
+            if (root == null) {
+                return null;
+            }
+            if (root == p || root == q) {
+                return root;
+            }
+
+            TreeNode left = lowestCommonAncestor(root.left, p, q);
+            TreeNode right = lowestCommonAncestor(root.right, p, q);
+
+            if (left != null && right != null) {
+                // 左右两边都不为空，那么我就是最近公共节点
+                return root;
+            }
+
+            if (left != null) {
+                return left;
+            }else {
+                return right;
+            }
+        }
+    }
+}
diff --git a/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode450.java b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode450.java
new file mode 100644
index 0000000..7af27fd
--- /dev/null
+++ b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode450.java
@@ -0,0 +1,50 @@
+package cn.whaifree.redo.redo_24_2_4;
+
+import cn.whaifree.leetCode.model.TreeNode;
+import org.junit.Test;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/2/4 0:26
+ * @注释
+ */
+public class LeetCode450 {
+
+    @Test
+    public void test() {
+        new Solution().deleteNode(TreeNode.constructTreeByArray(5,3,6,2,4,null,7), 7).printTree();
+
+    }
+
+    class Solution {
+        public TreeNode deleteNode(TreeNode root, int key) {
+            if (root==null) return null;
+
+            if (root.val == key) {
+                // 该节点要删除
+                if (root.right == null) {
+                    return root.left;
+                }
+
+                if (root.left == null) {
+                    return root.right;
+                }
+
+                TreeNode pre = root.right;
+                while (pre.left != null) {
+                    pre = pre.left;
+                }
+                pre.left = root.left;
+                return root.right;
+            }
+
+            if (root.val > key) {
+                root.left = deleteNode(root.left, key);
+            } else {
+                root.right = deleteNode(root.right, key);
+            }
+            return root;
+        }
+    }
+}
diff --git a/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode501.java b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode501.java
new file mode 100644
index 0000000..a4f5248
--- /dev/null
+++ b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode501.java
@@ -0,0 +1,73 @@
+package cn.whaifree.redo.redo_24_2_4;
+
+import cn.whaifree.leetCode.model.TreeNode;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.Consumer;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/2/3 23:28
+ * @注释
+ */
+public class LeetCode501 {
+
+    @Test
+    public void test() {
+        TreeNode root = TreeNode.constructTreeByArray(6,2,8,0,4,7,9,null,null,2,6);
+        for (int i : new Solution().findMode(root)) {
+            System.out.println(i);
+        }
+    }
+
+
+    class Solution {
+
+
+        TreeNode pre = null;
+        int most = 0;
+        int markCount = 0;
+        List<Integer> res = new ArrayList<>();
+        public int[] findMode(TreeNode root) {
+            recursion(root);
+            int[] re = new int[res.size()];
+            for (int i = 0; i < res.size(); i++) {
+                re[i] = res.get(i);
+            }
+            return re;
+        }
+
+        public void recursion(TreeNode root) {
+            if (root == null) {
+                return;
+            }
+            recursion(root.left);
+
+
+            if (pre == null) {
+                // 第一个节点走
+                markCount = 1;
+            } else if (pre.val == root.val) {
+                markCount++;
+            } else {
+                markCount = 1;
+            }
+
+            if (markCount > most) {
+                res.clear();
+                most = markCount;
+                res.add(root.val);
+            } else if (markCount == most) {
+                res.add(root.val);
+            }
+
+            pre = root;
+            recursion(root.right);
+        }
+    }
+
+
+}
diff --git a/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode701.java b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode701.java
new file mode 100644
index 0000000..5efe0c0
--- /dev/null
+++ b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode701.java
@@ -0,0 +1,65 @@
+package cn.whaifree.redo.redo_24_2_4;
+
+import cn.whaifree.leetCode.model.TreeNode;
+import org.junit.Test;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/2/4 12:18
+ * @注释
+ */
+public class LeetCode701 {
+
+    @Test
+    public void test() {
+
+        new Solution1().insertIntoBST(TreeNode.constructTreeByArray(4, 2, 7, 1, 3), 6).printTree();
+    }
+
+    class Solution {
+        public TreeNode insertIntoBST(TreeNode root, int val) {
+
+            if (root == null) {
+                return new TreeNode(val);
+            }
+
+            TreeNode pre = null;
+            TreeNode index = root;
+            while (index != null) {
+                pre = index;
+                if (index.val > val) {
+                    index = index.left;
+                } else {
+                    index = index.right;
+                }
+            }
+
+            if (pre.val > val) {
+                pre.left = new TreeNode(val);
+            } else {
+                pre.right = new TreeNode(val);
+            }
+
+            return root;
+        }
+    }
+
+    class Solution1 {
+        public TreeNode insertIntoBST(TreeNode root, int val) {
+
+            if (root == null) {
+                return new TreeNode(val);
+            }
+
+            if (root.val > val) {
+                root.left = insertIntoBST(root.left, val);
+            } else {
+                root.right = insertIntoBST(root.right, val);
+            }
+
+            return root;
+        }
+    }
+
+}
diff --git a/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode77.java b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode77.java
new file mode 100644
index 0000000..c525d1f
--- /dev/null
+++ b/src/main/java/cn/whaifree/redo/redo_24_2_4/LeetCode77.java
@@ -0,0 +1,72 @@
+package cn.whaifree.redo.redo_24_2_4;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/2/4 12:31
+ * @注释
+ */
+public class LeetCode77 {
+
+    @Test
+    public void test(
+    ) {
+        for (List<Integer> integers : new Solution().combine(5, 2)) {
+            System.out.println(integers);
+        }
+    }
+
+    class Solution {
+
+        List<List<Integer>> res = new ArrayList<>();
+        List<Integer> path = new ArrayList<>();
+
+        /**
+         * 返回范围 [1, n] 中所有可能的 k 个数的组合。
+         * @param n
+         * @param k
+         * @return
+         */
+        public List<List<Integer>> combine(int n, int k) {
+            recursion(1, n, k);
+            return res;
+        }
+
+        /**
+         * 从start 到 end 里选择一个数，放入path
+         * @param start 开始的数
+         * @param end 结束的数
+         * @param k
+         */
+        void recursion(int start, int end, int k) {
+
+
+
+            if (path.size() == k) {
+                res.add(new ArrayList<>(path));
+                return;
+            }
+
+            for (int i = start; i <= end; i++) {
+
+                // 剪枝
+                // 已经选择的数字size
+                // 还需选择的数k-size
+                // 可以选择的数 end-start
+                // 如果end-start<k-size 直接return
+                if (end - i + 1 < k - path.size()) {
+                    return;
+                }
+
+                path.add(i);
+                recursion(i + 1, end, k);
+                path.remove(path.size() - 1);
+            }
+        }
+    }
+}