[Leetcode]538. Convert BST to Greater Tree

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；

/**
 * 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) {
       helper(root);
       return root;      
    }        
    public void helper(TreeNode root){
        if (root!=null){
            helper(root.right);
            root.val+=sum;
            sum = root.val;
            helper(root.left);
        }
    }
}