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.

分析：递归。先判断根节点的val是否相同，如相同递归的判断左子树和右子树是否分别相同。代码如下：

class Solution {public:    bool isSameTree(TreeNode *p, TreeNode *q) {        if(p == NULL || q == NULL) return p == q;        if(p->val != q->val) return false;        return isSameTree(p->left, q->left) && isSameTree(p->right, q->right);    }};

Leetcode:Same Tree