image.png
image.png

解法

二叉树的中序遍历的反序，维护一个成员变量，记录遍历过程中遇到树节点值的加和。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int total = 0;
    public TreeNode convertBST(TreeNode root) {
        // 按右中左的顺序进行遍历，节点值进行加和      
        dfs(root);
        return root;                                           
    }


    public void dfs(TreeNode node) {
        if (node == null) {
            return;
        }
        dfs(node.right);        
        total += node.val;
        node.val = total;
        dfs(node.left);
    }
}