示例 1:
​
给定二叉树 [3,9,20,null,null,15,7]
​
    3
   / \
  9  20
    /  \
   15   7
返回 true 。
​
示例 2:
​
给定二叉树 [1,2,2,3,3,null,null,4,4]
​
       1
      / \
     2   2
    / \
   3   3
  / \
 4   4
返回 false 。

/**
 * 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 {
private:
    int deep(TreeNode* root)
    {
        if(!root) return 0;
        int r=1,l=1;
        if(!root->left && !root->right) return 1;
        if(root->left) l+=deep(root->left);
        if(root->right) r+=deep(root->right);
​
        if(l<0 || r<0 || abs(l-r) > 1) return INT_MIN;
        return max(l,r);
    }
public:
    bool isBalanced(TreeNode* root) {
        if(!root) return true;
   //     cout<<deep(root)<<endl;
        return deep(root)>0;
​
    }
};