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： 每个节点都加上比它大的，5+13 = 18， 2 + 5 + 13 = 20，13是最大的所以不用加
思路： 后续遍历，先遍历右子树，再遍历左子树 ，然后逐个往前加 后续遍历的结果是：[13,5,2] 13是最大的，不用加（相当于加0） 得到 [13, 5, 2] 5 加上前面的13, 得到 [13, 18, 2] 2 加上前面的18，得到[13, 18, 20]
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ public class Solution { int sum = 0; public TreeNode convertBST(TreeNode root) { if(root == null) { return root; } convertBST(root.right); root.val += sum; sum = root.val; convertBST(root.left); return root; } }