算法分析与设计——LeetCode Problem.538 Convert BST to Greater Tree

SYSU_Chan

于 2017-12-29 12:23:02 发布

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本文介绍了一种将二叉搜索树（BST）转换为Greater Tree的方法，即每个节点的值变为原值加上所有大于该值节点之和。通过后序遍历实现节点值的更新。

题目链接

问题描述

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

后序遍历BST然后把节点值逐个累加即可。

代码如下

class Solution {
public:
    TreeNode* convertBST(TreeNode* root) {
		if (root == NULL) return NULL;
		int sum = 0;
		backorder(root, sum);
		return root;
	}
private:
	void backorder(TreeNode *r, int &sum) {
		if (r != NULL) {
			backorder(r->right, sum);
			r->val += sum;
			sum = r->val;
			backorder(r->left, sum);
		}
	}
};