示例 1:
​
输入:       1         1
          / \       / \
         2   3     2   3
​
        [1,2,3],   [1,2,3]
​
输出: true
示例 2:
​
输入:      1          1
          /           \
         2             2
​
        [1,2],     [1,null,2]
​
输出: false
示例 3:
​
输入:       1         1
          / \       / \
         2   1     1   2
​
        [1,2,1],   [1,1,2]
​
输出: 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 {
public:
    bool isSameTree(TreeNode* p, TreeNode* q) {
        if(p==NULL && q==NULL) return true;
        else if(p==NULL ||q == NULL) return false;
        return p->val == q->val && isSameTree(p->left, q->left) && isSameTree(p->right, q->right);
    }
};