题目描述：
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.
解题思路：
递归调用比较左子树和右子树以及该节点。
代码描述：
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public boolean isSameTree(TreeNode p, TreeNode q) {
if( p == null && q == null)
return true;
else if(p == null || q == null)
return false;
else
return p.val == q.val && isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
}
}