给定一个二叉树，判断它是否是 平衡二叉树

示例 1：

输入：root = [3,9,20,null,null,15,7]
输出：true

我的解：

分治

/**
 * 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:
    int height(TreeNode* root) {
        if (root == NULL) {
            return 0;
        } else {
            return max(height(root->left), height(root->right)) + 1;
        }
    }

    bool isBalanced(TreeNode* root) {
        if (root == NULL) {
            return true;
        } else {
            return abs(height(root->left) - height(root->right)) <= 1 && isBalanced(root->left) && isBalanced(root->right);
        }
    }
};