Question：
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
1、题型分类：
2、思路：比较left.left与right.right和left.right与right.left,如此递归下去
3、时间复杂度：
4、代码：
public class Solution { public boolean isSymmetric(TreeNode root) { if(root==null) return true; return check(root.left,root.right); } public boolean check(TreeNode left,TreeNode right) { if(left==null && right ==null) return true; if(left==null || right==null) return false; return (left.val==right.val) && check(left.left, right.right) && check(left.right, right.left); } }
5、优化：
非递归
6、扩展：
原文：http://www.cnblogs.com/maydow/p/4645692.html