538.cpp

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