原题链接在这里：https://leetcode.com/problems/symmetric-tree/

题目：

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

But the following is not:

    1
   / \
  2   2
   \   \
   3    3

题解：

本题与Same Tree类似。这里比较是否symmetric, 也是用recursion, 需要写一个helper function, 递归调用，每次对称着比较。

Time Complexity: O(n), n是tree的node数目. Space: O(logn).

AC Java:

 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 public class Solution {
11     public boolean isSymmetric(TreeNode root) {
12         if(root == null){
13             return true;
14         }
15         return isSymm(root.left, root.right);
16     }
17     
18     private boolean isSymm(TreeNode p, TreeNode q){
19         if(p == null && q == null){
20             return true;
21         }
22         if(p == null || q == null){
23             return false;
24         }
25         return p.val == q.val && isSymm(p.left, q.right) && isSymm(p.right, q.left);
26     }
27 }