## [110\. Balanced Binary Tree](https://leetcode.com/problems/balanced-binary-tree/)

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

**Example 1:**


**Input:** root = \[3,9,20,null,null,15,7\]
**Output:** true

**Example 2:**


**Input:** root = \[1,2,2,3,3,null,null,4,4\]
**Output:** false

**Example 3:**

**Input:** root = \[\]
**Output:** true

**Constraints:**

- The number of nodes in the tree is in the range `[0, 5000]`.
- `-104 <= Node.val <= 104`

根據題意，左側樹高和右側樹高的差如果大於 1 即沒有平衡，使用遞迴解。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    bool isBalanced(TreeNode* root) {
        if(!root) return true;
        
        // 走訪左邊的樹
        int left = getHeight(root->left);
        
        // 走訪右邊的樹
        int right = getHeight(root->right);

        return abs(right - left) < 2 && isBalanced(root->left) && isBalanced(root->right);
    }
    int getHeight(TreeNode* root) {
        if(!root) return -1;
        // 每遞迴一次，height + 1
        return 1 + max(getHeight(root->left), getHeight(root->right));
    }
};

:::success
- 時間複雜度：$O(N)$
- 空間複雜度：$O(N)$
:::