力扣538. 把二叉搜索树转换为累加树

538. 把二叉搜索树转换为累加树 - 力扣（LeetCode）

右中左遍历

/**
 * 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 num = 0;//全局变量用来记录上一次的和
    public TreeNode convertBST(TreeNode root) {
        if(root==null) return root;//递归终止条件
        convertBST(root.right);//右
        root.val = root.val + num;//中
        num = root.val;
        convertBST(root.left);//左
        return root;

    }
}