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.

思路：同时递归两棵树，如果节点值不相等或者一棵树已经递归到头了而另一棵还没有，返回false;

1 /** 2  * Definition for a binary tree node. 3  * public class TreeNode { 4  *     int val; 5  *     TreeNode left; 6  *     TreeNode right; 7  *     TreeNode(int x) { val = x; } 8  * } 9  */10 class Solution {11     public boolean isSameTree(TreeNode p, TreeNode q) {12         if (p==null&&q==null)13             return true;                 //一开始如果传进两颗空树，返回true14         if ((p==null&&q!=null)||(p!=null&&q==null))15             return false;                //递归过程中，一棵树递归到头了，而另一颗没有16         if (p.val!=q.val)17             return false;18         return isSameTree(p.left,q.left)&&isSameTree(p.right,q.right); 19                                               // 同时递归20     }21 }