该题还有kmp、数哈希等方法，目前先放置，之后研究
    方法一：两次递归嵌套，注意递归结束的条件

    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 check(TreeNode* r, TreeNode* s){
    15.         if(!r&&!s){
    16.             return true;
    17.         }else if(!r||!s){
    18.             return false;
    19.         }
    20.         if(r->val!=s->val){
    21.             return false;
    22.         }
    23.         bool b1=check(r->left,s->left);
    24.         bool b2=check(r->right,s->right);
    25.         return b1&&b2;
    26.     }
    27.     bool dfs(TreeNode* r, TreeNode* s){
    28.         if(r==NULL){
    29.             return false;
    30.         }
    31.         bool a1=check(r,s);
    32.         bool a2=dfs(r->left,s);
    33.         bool a3=dfs(r->right,s);
    34.         return a1||a2||a3;
    35.     }
    36.     bool isSubtree(TreeNode* root, TreeNode* subRoot) {
    37.         return dfs(root,subRoot);
    38.     }
    39. };