[LC] 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

Example 1:

Input : {5,2,13}
              5
            /   
           2     13
Output : {18,20,13}
             18
            /   
          20     13

Example 2:

Input : {5,3,15}
              5
            /   
           3     15
Output : {20,23,15}
             20
            /   
          23     15

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: the root of binary tree
     * @return: the new root
     */
    public TreeNode convertBST(TreeNode root) {
        // write your code here
        int[] sum = new int[]{0};
        helper(root, sum);
        return root;
    }
    
    private void helper(TreeNode root, int[] sum) {
        if (root == null) {
            return;
        }
        helper(root.right, sum);
        sum[0] += root.val;
        root.val = sum[0];
        helper(root.left, sum);
    }
}