力扣-1038题 从二叉搜索树到更大和树（C++）- dfs

题目链接：https://leetcode-cn.com/problems/binary-search-tree-to-greater-sum-tree/
题目如下：

/**
 * 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* bstToGst(TreeNode* root) {
        dfs(root);
        return root;
    }
    void dfs(TreeNode* root){
        if(!root) return;

        dfs(root->right);
        
        sum_pre+=root->val;
        root->val=sum_pre;
        
        dfs(root->left);

    }
private:
    int sum_pre=0;
};