方法：
    遇到问题：pair的使用

    1. /**
    2.  * Definition for a binary tree node.
    3.  * struct TreeNode {
    4.  *     int val;
    5.  *     TreeNode *left;
    6.  *     TreeNode *right;
    7.  *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
    8.  *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    9.  *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
    10.  * };
    11.  */
    12. class Solution {
    13. public:
    14.     bool isBalanced(TreeNode* root) {
    15.         pair<bool,int> p=check(root);
    16.         return p.first;
    18.     }
    19.     pair<bool,int> check(TreeNode* root)
    20.     {
    21.         pair<bool,int>result;
    22.         if(root==NULL){
    23.             return result=make_pair(true,0);
    24.         }
    25.         pair<bool,int>result1;
    26.         pair<bool,int>result2;
    27.         int codepth;
    28.         int depth;
    29.         bool ret;     
    30.         result1=check(root->left);        
    31.         result2=check(root->right);        
    32.         depth=result1.second>result2.second?result1.second:result2.second;
    33.         depth+=1;
    34.         codepth=abs(result1.second-result2.second);
    35.         if(codepth<2&&(result1.first&&result2.first)){
    36.             ret=true;
    37.         }
    38.         else{
    39.             ret=false;
    40.         }
    41.         result=make_pair(ret,depth);
    42.         return result;
    43.     }
    44. };