代码随想录算法训练营第22天（二叉树9) | 669. 修剪二叉搜索树 108.将有序数组转换为二叉搜索树 538.把二叉搜索树转换为累加树-拓冰建站

669. 修剪二叉搜索树

leetcode题目链接
题目链接/文章讲解
视频讲解
这道题目比较难，比添加增加和删除节点难的多，建议先看视频理解。

//递归法
class Solution {
public:TreeNode* trimBST(TreeNode* root, int low, int high) {if (root == nullptr ) return nullptr;if (root->val < low) {TreeNode* right = trimBST(root->right, low, high); // 寻找符合区间[low, high]的节点return right;}if (root->val > high) {TreeNode* left = trimBST(root->left, low, high); // 寻找符合区间[low, high]的节点return left;}root->left = trimBST(root->left, low, high); // root->left接入符合条件的左孩子root->right = trimBST(root->right, low, high); // root->right接入符合条件的右孩子return root;}
};
//迭代法
class Solution {
public:TreeNode* trimBST(TreeNode* root, int L, int R) {if (!root) return nullptr;// 处理头结点，让root移动到[L, R] 范围内，注意是左闭右闭while (root != nullptr && (root->val < L || root->val > R)) {if (root->val < L) root = root->right; // 小于L往右走else root = root->left; // 大于R往左走}TreeNode *cur = root;// 此时root已经在[L, R] 范围内，处理左孩子元素小于L的情况while (cur != nullptr) {while (cur->left && cur->left->val < L) {cur->left = cur->left->right;}cur = cur->left;}cur = root;// 此时root已经在[L, R] 范围内，处理右孩子大于R的情况while (cur != nullptr) {while (cur->right && cur->right->val > R) {cur->right = cur->right->left;}cur = cur->right;}return root;}
};

108.将有序数组转换为二叉搜索树

leetcode题目链接
题目链接/文章讲解
视频讲解

//递归法
class Solution {
private:TreeNode* traversal(vector<int>& nums, int left, int right) {if (left > right) return nullptr;int mid = left + ((right - left) / 2);TreeNode* root = new TreeNode(nums[mid]);root->left = traversal(nums, left, mid - 1);root->right = traversal(nums, mid + 1, right);return root;}
public:TreeNode* sortedArrayToBST(vector<int>& nums) {TreeNode* root = traversal(nums, 0, nums.size() - 1);return root;}
};
//迭代法
class Solution {
public:TreeNode* sortedArrayToBST(vector<int>& nums) {if (nums.size() == 0) return nullptr;TreeNode* root = new TreeNode(0);   // 初始根节点queue<TreeNode*> nodeQue;           // 放遍历的节点queue<int> leftQue;                 // 保存左区间下标queue<int> rightQue;                // 保存右区间下标nodeQue.push(root);                 // 根节点入队列leftQue.push(0);                    // 0为左区间下标初始位置rightQue.push(nums.size() - 1);     // nums.size() - 1为右区间下标初始位置while (!nodeQue.empty()) {TreeNode* curNode = nodeQue.front();nodeQue.pop();int left = leftQue.front(); leftQue.pop();int right = rightQue.front(); rightQue.pop();int mid = left + ((right - left) / 2);curNode->val = nums[mid];       // 将mid对应的元素给中间节点if (left <= mid - 1) {          // 处理左区间curNode->left = new TreeNode(0);nodeQue.push(curNode->left);leftQue.push(left);rightQue.push(mid - 1);}if (right >= mid + 1) {         // 处理右区间curNode->right = new TreeNode(0);nodeQue.push(curNode->right);leftQue.push(mid + 1);rightQue.push(right);}}return root;}
};

538.把二叉搜索树转换为累加树

leetcode题目链接

题目链接/文章讲解
视频讲解
本题也不难，在求二叉搜索树的最小绝对差和众数那两道题目都讲过了双指针法，思路是一样的。

//递归法
class Solution {
private:int pre = 0; // 记录前一个节点的数值void traversal(TreeNode* cur) { // 右中左遍历if (cur == NULL) return;traversal(cur->right);cur->val += pre;pre = cur->val;traversal(cur->left);}
public:TreeNode* convertBST(TreeNode* root) {pre = 0;traversal(root);return root;}
};
//迭代法
class Solution {
private:int pre; // 记录前一个节点的数值void traversal(TreeNode* root) {stack<TreeNode*> st;TreeNode* cur = root;while (cur != NULL || !st.empty()) {if (cur != NULL) {st.push(cur);cur = cur->right;   // 右} else {cur = st.top();     // 中st.pop();cur->val += pre;pre = cur->val;cur = cur->left;    // 左}}}
public:TreeNode* convertBST(TreeNode* root) {pre = 0;traversal(root);return root;}
};