【LeetCode】110. Balanced Binary Tree(Easy)解题报告

【LeetCode】110. Balanced Binary Tree(Easy)解题报告

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

题目描述：

  Given a binary tree, determine if it is height-balanced.

  For this problem, a height-balanced binary tree is defined as : a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example 1:

Given the following tree [3,9,20,null,null,15,7]:

3
/ \
9  20
/  \
15   7
Return true.

Example 2:

Given the following tree [1,2,2,3,3,null,null,4,4]:

1
/ \
2   2
/ \
3   3
/ \
4   4
Return false.

Solution:

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
判断是不是高度平衡，左右子树高度差值是否大于一
后序遍历判断左右子树高度
time : O(n)
space : O(n)
*/
class Solution {
public boolean isBalanced(TreeNode root) {
if(root == null) return true;
return helper(root)!=-1;
}
public int helper(TreeNode root){
if(root == null) return 0;
int l = helper(root.left);
int r = helper(root.right);
if(l == -1 || r == -1 || Math.abs(l-r)>1){
return -1;
}
return Math.max(l,r)+1;
}
}

Date：2018年3月17日