【LeetCode】Binary Tree Tilt 解题报告

标签（空格分隔）： LeetCode

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题目地址：https://leetcode.com/problems/binary-tree-tilt/#/description

题目描述：

Given a binary tree, return the tilt of the whole tree.

The tilt of a tree node is defined as the absolute difference between the sum of all left subtree node values and the sum of all right subtree node values. Null node has tilt 0.

The tilt of the whole tree is defined as the sum of all nodes’ tilt.

Example 1:

Input:          1       /   \      2     3Output: 1Explanation: Tilt of node 2 : 0Tilt of node 3 : 0Tilt of node 1 : |2-3| = 1Tilt of binary tree : 0 + 0 + 1 = 1

Note:

1. The sum of node values in any subtree won’t exceed the range of 32-bit integer.
2. All the tilt values won’t exceed the range of 32-bit integer.

Ways

这个题很容易想到dfs，但是怎么写呢。这个用到的是后序遍历，用一个res来保存左右差的结果，函数的返回值是左右子树的和加上根节点的值。这样统计之后就能求出所有左右子树的和的差值。

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    int res = 0;    public int findTilt(TreeNode root) {        postOrder(root);        return res;    }    public int postOrder(TreeNode root){        if(root == null){            return 0;        }        int left = postOrder(root.left);        int right = postOrder(root.right);        res += Math.abs(left - right);        return left + right + root.val;    }}

Date

2017 年 5 月 9 日