diff --git a/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode17.java b/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode17.java
new file mode 100644
index 0000000..29cb0f7
--- /dev/null
+++ b/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode17.java
@@ -0,0 +1,70 @@
+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 2024/2/2 10:31
+ * @注释
+ */
+public class LeetCode17 {
+
+    @Test
+    public void test() {
+        String digits = "";
+        new Solution().letterCombinations(digits).forEach(s -> System.out.println(s));
+    }
+
+    class Solution {
+        List<String> map = new ArrayList<>();
+
+        List<String> result = new ArrayList<>();
+
+        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, digits.length());
+            return result;
+        }
+
+        /**
+         *
+         * @param digits 原始字符 23
+         * @param number 使用到第几个字符
+         * @param needLength 需要几个字符，等价于digits.length
+         */
+        void backTracking(String digits, int number,int needLength) {
+            if (s.length() == needLength) {
+                result.add(new String(s.toString()));
+                return;
+            }
+
+            int c = digits.charAt(number) - 48;
+            String sValue = map.get(c);
+            int length = sValue.length();
+            for (int i = 0; i < length; i++) {
+                char c1 = sValue.charAt(i);
+                s.append(c1);
+                backTracking(digits, number + 1, needLength);
+                s.deleteCharAt(s.length() - 1);
+            }
+        }
+    }
+}
diff --git a/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode216.java b/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode216.java
new file mode 100644
index 0000000..2f7b01a
--- /dev/null
+++ b/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode216.java
@@ -0,0 +1,81 @@
+package cn.whaifree.leetCode.BackTracking;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/2/2 8:22
+ * @注释
+ */
+public class LeetCode216 {
+
+    @Test
+    public void test() {
+        Solution solution = new Solution();
+        solution.combinationSum3(9, 45).forEach(
+                list -> {
+                    System.out.println(list);
+                }
+        );
+    }
+
+    class Solution {
+
+        List<List<Integer>> res = new LinkedList<>();
+        List<Integer> path = new ArrayList<>();
+        int sum = 0;
+        /**
+         * 相加之和为n的k个数的组合
+         * @param k
+         * @param n
+         * @return
+         */
+        public List<List<Integer>> combinationSum3(int k, int n) {
+            circle(1, 9, n, k);
+            return res;
+        }
+
+        public void circle(int start, int end, int n, int k) {
+            if (path.size() == k && sum == n) {
+                res.add(new ArrayList<>(path));
+                return;
+            }
+
+            // 1. 如果sum>n了，证明往后加只会更大，因为都是正数，就不再继续了
+            // sum>n
+            //
+            // 2. 如果 9个数要9个数
+            // 已经选择size
+            // 还需选择k-size
+            // 可以选择的数end-start
+            // 可以选择的数<还需选择的数 end-start<k-size
+
+            for (int i = start; i <= end ; i++) {
+                // 可以选择的数<还需选择的数
+                if (end - i + 1 < k - path.size()) {
+                    return;
+                }
+                path.add(i);
+                sum += i;
+
+                // 如果 加上某个数已经超过了n，那么剩下的正数都没必要再加了。如1234，后面只能选5678，如果去了4加了5678必然会超过n
+                if (sum > n) {
+                    path.remove(path.size() - 1);
+                    sum -= i;
+                    return;
+                }
+                circle(i + 1, end, n, k);
+                path.remove(path.size() - 1);
+                sum -= i;
+            }
+        }
+    }
+
+
+
+}
diff --git a/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode77.java b/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode77.java
new file mode 100644
index 0000000..1524f8c
--- /dev/null
+++ b/src/main/java/cn/whaifree/leetCode/BackTracking/LeetCode77.java
@@ -0,0 +1,57 @@
+package cn.whaifree.leetCode.BackTracking;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/1/31 20:24
+ * @注释
+ */
+public class LeetCode77 {
+
+    @Test
+    public void test() {
+        new Solution().combine(4, 4).forEach(
+                list -> {
+                    list.forEach(System.out::print);
+                    System.out.println();
+                }
+        );
+    }
+
+
+    class Solution {
+        List<List<Integer>> res = new LinkedList<>();
+
+        List<Integer> path = new ArrayList<>();
+
+        public List<List<Integer>> combine(int n, int k) {
+            circle(1, n, k);
+            return res;
+        }
+
+        void circle(int start, int end, int k) {
+            if (path.size() == k) {
+                res.add(new ArrayList<>(path));
+                return;
+            }
+
+            // 剪枝，可以选择的个数不满足最小需要的个数
+            // 已经选择的个数 path.size
+            // 还需要的个数 k-path.size
+            // 可以选择的个数<还需要的个数 end-i<k-path.size 时 退出
+            // 即 end-i>k-path.size时正常执行 i<end-(k-path.size) 时正常执行
+            for (int i = start; i <= end - (k - path.size()) + 1; i++) {
+                path.add(i);
+                circle(i + 1, end, k);
+                path.remove(path.size() - 1);
+            }
+        }
+
+    }
+}
diff --git a/src/main/java/cn/whaifree/leetCode/Tree/LeetCode108.java b/src/main/java/cn/whaifree/leetCode/Tree/LeetCode108.java
new file mode 100644
index 0000000..b7c73c2
--- /dev/null
+++ b/src/main/java/cn/whaifree/leetCode/Tree/LeetCode108.java
@@ -0,0 +1,40 @@
+package cn.whaifree.leetCode.Tree;
+
+import cn.whaifree.leetCode.model.TreeNode;
+import org.junit.Test;
+
+import java.util.Deque;
+import java.util.LinkedList;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/1/31 18:56
+ * @注释
+ */
+public class LeetCode108 {
+
+    @Test
+    public void test() {
+        new Solution().sortedArrayToBST(new int[]{1, 2, 3, 4, 5, 6, 7, 8}).printTree();
+    }
+
+    class Solution {
+        public TreeNode sortedArrayToBST(int[] nums) {
+            return construct(nums, 0, nums.length - 1);
+        }
+
+        public TreeNode construct(int[] nums, int start, int end) {
+            if (start > end) {
+                return null;
+            }
+            int middle = (end + start) / 2;
+            int num = nums[middle];
+            TreeNode root = new TreeNode(num);
+            root.left = construct(nums, start, middle - 1);
+            root.right = construct(nums, middle + 1, end);
+            return root;
+        }
+    }
+
+}
diff --git a/src/main/java/cn/whaifree/leetCode/Tree/LeetCode450.java b/src/main/java/cn/whaifree/leetCode/Tree/LeetCode450.java
new file mode 100644
index 0000000..acae216
--- /dev/null
+++ b/src/main/java/cn/whaifree/leetCode/Tree/LeetCode450.java
@@ -0,0 +1,157 @@
+package cn.whaifree.leetCode.Tree;
+
+import cn.whaifree.leetCode.model.TreeNode;
+import org.junit.Test;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/1/31 9:38
+ * @注释
+ */
+public class LeetCode450 {
+
+    @Test
+    public void test() {
+        new Solution1().deleteNode(TreeNode.constructTreeByArray(5,3,6,2,4,null,7,null,null), 0).printTree();
+
+    }
+
+    class Solution {
+        public TreeNode deleteNode(TreeNode root, int key) {
+            if (root == null) {
+                return null;
+            }
+
+            if (root.left == null && root.right == null && root.val == key) {
+                return null;
+            }
+
+
+            TreeNode pre = null;
+            TreeNode tmp = root;
+            while (tmp!=null) {
+                if (tmp.val > key) {
+                    pre = tmp;
+                    tmp = tmp.left;
+                } else if (tmp.val < key) {
+                    pre = tmp;
+                    tmp = tmp.right;
+                } else {
+                    break;
+                }
+            }
+
+            // 找不到该要删除的节点
+            if (pre != null && pre.val != key && tmp == null) {
+                return root;
+            }
+
+
+
+            // 左右都为空
+            // 左为空
+            // 右为空
+            if (pre.left == tmp) {
+                if (tmp.right == null && tmp.left == null) {
+                    pre.left = null;
+                } else if (tmp.left == null) {
+                    pre.left = tmp.right;
+                } else if (tmp.right == null) {
+                    pre.left = tmp.left;
+                }
+            }else if (pre.right == tmp){
+                if (tmp.right == null && tmp.left == null) {
+                    pre.right = null;
+                } else if (tmp.left == null) {
+                    pre.right = tmp.right;
+                } else if (tmp.right == null) {
+                    pre.right = tmp.left;
+                }
+            }
+            // 左右都不空
+            // tmp.left移动到tmp.right.left.....最后
+            TreeNode move = tmp.right;
+            while (move.left != null) {
+                move = move.left;
+            }
+            move.left = tmp.left;
+            // tmp.right.left移动到tmp的位置 有可能在pre的左边或右边
+            if (pre.left == tmp) {
+                pre.left = tmp.right;
+            } else {
+                pre.right = tmp.right;
+            }
+            return root;
+        }
+    }
+
+
+    class Solution1 {
+        /**
+         * 递归
+         * @param root
+         * @param key
+         * @return
+         */
+        public TreeNode deleteNode(TreeNode root, int key) {
+
+            if (root == null) {
+                return root;
+            }
+            if (root.val == key) {
+
+                // 左边为空 返回右边
+                // 两边都为空，则返回空，随便返回子节点
+                if (root.left == null) {
+                    return root.right;
+                }
+                if (root.right == null) {
+                    return root.left;
+                }
+                TreeNode index = root.right;
+                while (index.left != null) {
+                    index = index.left;
+                }
+                index.left = root.left;
+                return root.right;
+            }
+
+            if (root.val > key) {
+                root.left = deleteNode(root.left, key);
+            } else if (root.val < key) {
+                root.right = deleteNode(root.right, key);
+            }
+            return root;
+        }
+    }
+
+    /**
+     * 把二叉树变成链表
+     */
+    class Solution2 {
+        TreeNode pre = new TreeNode();
+        TreeNode cur = pre;
+
+        public TreeNode deleteNode(TreeNode root, int key) {
+            search(root, key);
+            return pre.right;
+        }
+
+        private void search(TreeNode root, int key) {
+            if (root == null) {
+                return;
+            }
+            search(root.left, key);
+            TreeNode r = root.right;
+            if (root.val != key) {
+                cur.right = root;
+                root.left = null;
+                root.right = null;
+                cur = cur.right;
+            }
+            search(r, key);
+
+        }
+    }
+}
diff --git a/src/main/java/cn/whaifree/leetCode/Tree/LeetCode538.java b/src/main/java/cn/whaifree/leetCode/Tree/LeetCode538.java
new file mode 100644
index 0000000..399908f
--- /dev/null
+++ b/src/main/java/cn/whaifree/leetCode/Tree/LeetCode538.java
@@ -0,0 +1,65 @@
+package cn.whaifree.leetCode.Tree;
+
+import cn.whaifree.leetCode.model.TreeNode;
+import org.junit.Test;
+
+import java.util.Deque;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/1/31 19:15
+ * @注释
+ */
+public class LeetCode538 {
+
+    @Test
+    public void test() {
+
+        new Solution1().convertBST(TreeNode.constructTreeByArray(4, 1, 6, 0, 2, 5, 7, null, null, null, 3, null, null, null, 8)).printTree();
+    }
+
+    class Solution {
+        int sum = 0;
+        public TreeNode convertBST(TreeNode root) {
+            if (root == null) {
+                return null;
+            }
+            root.right = convertBST(root.right);
+            root.val += sum;
+            sum = root.val;
+            root.left = convertBST(root.left);
+            return root;
+        }
+    }
+
+
+    class Solution1 {
+        public TreeNode convertBST(TreeNode root) {
+
+            if (root == null) {
+                return null;
+            }
+
+            Deque<TreeNode> stack = new java.util.LinkedList<>();
+            stack.push(root);
+
+
+            int sum = 0;
+            while (!stack.isEmpty()) {
+                TreeNode pop = stack.pop();
+                if (pop == null) {
+                    TreeNode ans = stack.pop();
+                    ans.val += sum;
+                    sum = ans.val;
+                } else {
+                    if (pop.left!=null) stack.push(pop.left);
+                    stack.push(pop);
+                    stack.push(null);
+                    if (pop.right!=null) stack.push(pop.right);
+                }
+            }
+            return root;
+        }
+    }
+}
diff --git a/src/main/java/cn/whaifree/leetCode/Tree/LeetCode669.java b/src/main/java/cn/whaifree/leetCode/Tree/LeetCode669.java
new file mode 100644
index 0000000..d93ce75
--- /dev/null
+++ b/src/main/java/cn/whaifree/leetCode/Tree/LeetCode669.java
@@ -0,0 +1,40 @@
+package cn.whaifree.leetCode.Tree;
+
+import cn.whaifree.leetCode.model.TreeNode;
+import org.junit.Test;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/1/31 14:01
+ * @注释
+ */
+public class LeetCode669 {
+
+    @Test
+    public void test() {
+        new Solution().trimBST(TreeNode.constructTreeByArray(3,0,4,null,2,null,null,1), 1, 3).printTree();
+    }
+
+    class Solution {
+
+        public TreeNode trimBST(TreeNode root, int low, int high) {
+            if (root == null) {
+                return null;
+            }
+
+            root.left = trimBST(root.left, low, high);
+            root.right = trimBST(root.right, low, high);
+
+            if (root.val < low) {
+                return root.right;
+            }
+
+            if (root.val > high) {
+                return root.left;
+            }
+
+            return root;
+        }
+    }
+}
diff --git a/src/main/java/cn/whaifree/leetCode/Tree/LeetCode701.java b/src/main/java/cn/whaifree/leetCode/Tree/LeetCode701.java
new file mode 100644
index 0000000..fa80ada
--- /dev/null
+++ b/src/main/java/cn/whaifree/leetCode/Tree/LeetCode701.java
@@ -0,0 +1,88 @@
+package cn.whaifree.leetCode.Tree;
+
+import cn.whaifree.leetCode.model.TreeNode;
+import org.junit.Test;
+
+/**
+ * @version 1.0
+ * @Author whai文海
+ * @Date 2024/1/31 9:13
+ * @注释
+ */
+public class LeetCode701 {
+
+    @Test
+    public void test() {
+        new Solution2().insertIntoBST(TreeNode.constructTreeByArray(5,null,14,10,77,null,null,null,95,null,null), 4).printTree();
+
+    }
+
+    class Solution {
+        public TreeNode insertIntoBST(TreeNode root, int val) {
+            if (root == null) {
+                return new TreeNode(val);
+            }
+            insert(root, val);
+            return root;
+        }
+
+        public void insert(TreeNode root, int val) {
+            if (root == null) {
+                return;
+            }
+
+
+            if (root.val < val) {
+                insert(root.right, val);
+                if (root.right == null) {
+                    root.right = new TreeNode(val);
+                }
+            } else {
+                insert(root.left, val);
+                if (root.left == null) {
+                    root.left = new TreeNode(val);
+                }
+            }
+        }
+    }
+
+    class Solution1 {
+        public TreeNode insertIntoBST(TreeNode root, int val) {
+            // 找到了叶子节点，该位置就是要插入的地方
+            if (root == null) {
+                return new TreeNode(val);
+            }
+
+            if (root.val < val) {
+                root.right = insertIntoBST(root.right, val);
+            }else if (root.val > val){
+                root.left = insertIntoBST(root.left, val);
+            }
+            return root;
+        }
+    }
+    class Solution2 {
+        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;
+
+
+        }
+    }
+
+}