LeetCode538-20.9.21-二叉搜索树转换为累加树

题目链接：
LeetCode538
思路：dfs反中序遍历树
看题解收获：morris减少空间复杂度
代码：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private int sum=0;
    public TreeNode convertBST(TreeNode root) {
        dfs(root);
        return root;
    }
    public void dfs(TreeNode node){
        if(node==null)return;
        dfs(node.right);
        sum+=node.val;
        node.val=sum;
        dfs(node.left);
    }
}