代码随想录算法训练营第二十三天| 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

669. 修剪二叉搜索树

题目链接
文章讲解

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* trimBST(TreeNode* root, int low, int high) {
        if (!root) return nullptr;
        if (root->val < low) return trimBST(root->right, low, high);
        if (root->val > high) return trimBST(root->left, low, high);
        root->left = trimBST(root->left, low, high);
        root->right = trimBST(root->right, low, high);
        return root;
    }
};

108.将有序数组转换为二叉搜索树

题目链接
文章讲解

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    TreeNode* traversal(vector<int>& nums, int begin, int end) {
        if (begin == end) return nullptr;
        int mid = (begin + end) >> 1;
        TreeNode* root = new TreeNode(nums[mid]);
        root->left = traversal(nums, begin, mid);
        root->right = traversal(nums, mid + 1, end);
        return root;
    }

    TreeNode* sortedArrayToBST(vector<int>& nums) {
        return traversal(nums, 0, nums.size());
    }
};

538.把二叉搜索树转换为累加树

题目链接
文章讲解

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void traversal(TreeNode* root, int& pre) {
        if (!root)
            return;
        traversal(root->right, pre);
        pre = root->val += pre;
        traversal(root->left, pre);
    }

    TreeNode* convertBST(TreeNode* root) {
        int pre = 0;
        traversal(root, pre);
        return root;
    }
};