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) {}
 * };
 */
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;
		if((p == NULL&&q != NULL)||(p != NULL&&q == NULL)||(p->val != q->val)) return false;
		return isSameTree(p->left,q->left)&&isSameTree(p->right,q->right);
    }
};