java数据结构与算法刷题-----LeetCode538. 把二叉搜索树转换为累加树
/**
* 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 sum = 0;//指向逆中序遍历（右中左）前一个结点的val值
public TreeNode convertBST(TreeNode root) {
if(root == null) return null;
convertBST(root.right);//先右子树
sum += root.val;//累加，这就是当前结点的值，也是下一个结点的前驱值
root.val = sum;//然后保存累加后的值
convertBST(root.left);//然后左子树
return root;
}
}