方法一：深度优先遍历

    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 isSameTree(TreeNode* p, TreeNode* q) {
    15.         if(p==NULL&&q==NULL){
    16.             return ret;
    17.         }
    18.         else if((p!=NULL&&q==NULL)||(p==NULL&&q!=NULL)){
    19.             return false;
    20.         }
    21.         if(p->val==q->val){
    22.         }
    23.         else{
    24.             ret=false;
    25.         }
    26.         ret=isSameTree(p->left,q->left);
    27.         ret=isSameTree(p->right,q->right);
    28.         return ret;
    29.     }
    30.     bool ret=true;
    31. };