https://programmercarl.com/0538.%E6%8A%8A%E4%BA%8C%E5%8F%89%E6%90%9C%E7%B4%A2%E6%A0%91%E8%BD%AC%E6%8D%A2%E4%B8%BA%E7%B4%AF%E5%8A%A0%E6%A0%91.html

视频讲解：https://www.bilibili.com/video/BV1d44y1f7wP

CPP

/**
 * 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:
    int pre = 0;
    void traversal(TreeNode* node){
        
        if(node == NULL)return ;
        traversal(node->right);
        node->val += pre;
        pre = node->val;
        traversal(node->left);
        return ;
        
    }
    TreeNode* convertBST(TreeNode* root) {
        traversal(root);
        return root;
    }
};