[LeetCode]第十六题 ：对称树

题目描述：

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

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

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

But the following [1,2,2,null,3,null,3] is not:

1 / \ 2 2 \ \ 3 3

Note:Bonus points if you could solve it both recursively and iteratively.

题目解释：

给出一个二叉树，判断它是否是对称的。

题目解法：

1.我的解法。首先树肯定要用递归；判断左子树和右子树是否对称，先判断左子树的结构和右子树的结构是否相同，在判断值是否相同；最后递归调用即可。代码如下：

/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ class Solution { public boolean isSymmetric(TreeNode root) { if(root == null) return true; return a(root.left,root.right); } private boolean a(TreeNode left,TreeNode right) { if(left == null && right == null) return true; if((left == null && right != null) || (left != null && right == null)) return false; if(left.val != right.val) return false; return true && a(left.right,right.left) && a(left.left,right.right); } }