Given two binary trees, write a function to check if they are equal or not.

Two binary trees are considered equal if they are structurally identical and the nodes have the same value.

/**
 * Definition for binary tree
 * 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->val==q->val && isSameTree(p->left,q->left) && isSameTree(p->right,q->right))
        return true;
        else return false;
        */
        //return isSameTree(p->left,q->left);
        //return isSameTree(p->right,q->right);
        if(p==NULL && q==NULL)return true;//两者都是NULL
        if(p!=NULL && q!=NULL && p->val==q->val//两者结点数据相等
           && isSameTree(p->left,q->left) //且左子树相同
           && isSameTree(p->right,q->right))//且右子树相同
            return true;
        else return false;
    }
};