[leetcode]44 Balanced Binary Tree

题目链接：https://leetcode.com/problems/balanced-binary-tree/

Runtimes：21ms

1、问题

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.

2、分析

首先要理解什么是平衡二叉树，即所有节点的左右子树的高度相差小于2，就是平衡的。本来想用非递归的，不过写出来好麻烦，最终还是用了递归，没想到跑过了，哈哈。

3、小结

其实只要递归中操作不多，就不会time out，get新技能。只要检查出不是平衡二叉树，直接就返回了，速度好快！

4、实现

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
int isBalanced(TreeNode *root, bool &f)
{
if(!f)
return -1;
if(root == NULL)
return -1;
int leftH, rightH;
leftH = isBalanced(root->left, f) + 1;
rightH = isBalanced(root->right, f) + 1;
if(abs(leftH - rightH) > 1)
f = false;
return leftH < rightH ? rightH : leftH;
}

bool isBalanced(TreeNode *root) {
bool f = true;
isBalanced(root, f);
if(f)
return true;
else
return false;
}
};

5、反思

简单题目，效果不错。