Add to List 538. Convert BST to Greater Tree

Posted 2020-10-11 nullptr

Given a Binary Search Tree (BST), convert it to a Greater Tree such that every key of the original BST is changed to the original key plus sum of all keys greater than the original key in BST.

Example:

Input: The root of a Binary Search Tree like this:
              5
            /              2     13

Output: The root of a Greater Tree like this:
             18
            /             20     13

思路：中序遍历；然后关键点在于用了一个全局变量；

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 class Solution{
11     private int sum = 0;
12     public TreeNode convertBST(TreeNode root) {
13        helper(root);
14        return root;      
15     }        
16     public void helper(TreeNode root){
17         if (root!=null){
18             helper(root.right);
19             root.val+=sum;
20             sum = root.val;
21             helper(root.left);
22         }
23     }
24 }