# 【LeetCode】 110. Balanced Binary Tree

## Description
> 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.

> 給一棵二元樹，判斷它是不是平衡樹。

> 本題目對於一棵平衡樹的定義為：
> > 對於每個節點，它左右子樹的高度相差必須小於等於一。

## Example:

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

* 用遞迴求樹高，然後將左右子樹樹高差距做判斷，如果符合就以它們為樹根繼續做遞迴。

### Code

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int FindHeight(TreeNode* node)
    {
        if(!node) return 0;
        else return max(FindHeight(node->left),FindHeight(node->right)) + 1;
    }
    bool isBalanced(TreeNode* root) {
        if(!root) return true;
        if(abs(FindHeight(root->left) - FindHeight(root->right)) > 1) return false;
        return isBalanced(root->left)&&isBalanced(root->right);
    }
};

###### tags: `LeetCode` `C++`