LeetCode-Convert BST to Greater Tree

Description：

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
           

题意：对于一颗二叉搜索树，对于每个结点累加上这颗树中比当前结点大的值；

解法：我们知道一颗二叉搜索树中，root.left.val < root.val < root.right.val；因此，问题就转化为了如何遍历一颗二叉树，遍历次序为root.right -> root -> root.left；同时，我们用一个变量记录遍历过程中的所有值得和，这样在返回计算上一个值时进行累加得值就是这个统计值；

Java

/**
 * 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) {
        if (root == null) return root;
        convertBST(root.right);
        sum += root.val;
        root.val = sum;
        convertBST(root.left);
        return root;
    }
}