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

Given a binary search Tree ｀{5,2,3}｀:

              5
            /   
           2     13

Return the root of new tree

             18
            /   
          20     13

逆中序遍历树，注意root的greatSum是input greatSum和右子树sum的和；输入给左子树的greatSum 是root.val

/**
 * 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
        if (root == null) {
            return root;
        }
        helper(root, 0);
        return root;
    }
    private int helper(TreeNode root, int greatSum){
        int sum = 0;
        if (root.right != null) {
            sum += helper(root.right, greatSum);
        }
        int temp = greatSum + sum;
        sum += root.val;
        root.val += temp;
        if (root.left != null) {
            sum += helper(root.left, root.val);
        }
        return sum;
    }
}