*Leetcode 538. 把二叉搜索树转换为累加树

https://leetcode.cn/problems/convert-bst-to-greater-tree/

按树的遍历逻辑梳理清楚即可

/**
 * 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* dfs(TreeNode*root, int &cur) {
        // cur 当前已经搜索过的结点大于等于当前节点值的sum 
        // val 已经遍历节点加当前结点
        
        if(!root) {
            cur = cur;
            return root;
        }
        // int now = cur;
        dfs(root->right, cur);  
        root->val += cur;
        cur = root->val;
        dfs(root->left, cur);
        // cur = lv;
        return root;
    }


    TreeNode* convertBST(TreeNode* root) {
        int v = 0;
        return dfs(root, v);
    }
};