https://leetcode.com/problems/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
解题思路：
右子树->root->左子树的顺序，从下往上开始处理。
对于每个节点，记录往前的累加和sum，在置到root上。
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ class Solution { public TreeNode convertBST(TreeNode root) { int[] sum = new int[1]; helper(root, sum); return root; } public void helper(TreeNode root, int[] sum) { if (root ==null) { return; } helper(root.right, sum); root.val += sum[0]; sum[0] = root.val; helper(root.left, sum); } }
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