Leetcode——563. Binary Tree Tilt

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:

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

Note:

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

解题思路

给定一个二叉树，求解二叉树各个节点的左右子树的绝对值t，整棵树的节点的绝对值tilt等于各个节点的绝对值t之和

用递归

• 如果当前节点为null，则返回0
• 得到当前节点左子树的绝对值和
• 得到当前节点右子树的绝对值和
• 增加当前节点的左右孩子的绝对值
• 返回当前节点的左右孩子的绝对值 + 自己的节点值，用于当前节点父节点的计算

AC代码

class Solution {

    int tilt = 0;
    public int findTilt(TreeNode root) {
        sum(root);
        return tilt;

    }
    public int sum(TreeNode root) {
        // TODO Auto-generated method stub
        if(root == null) return 0;
        int left = sum(root.left);
        int right = sum(root.right);
        tilt += left > right ? left - right : right - left;
        return left + right + root.val; 
    }
}