题目来源：https://leetcode.com/problems/symmetric-tree/

问题描述

101. Symmetric Tree

Easy

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.

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题意

判断二叉树是否对称。

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思路

递归方法treeSymmetric输入两棵子树的根节点r1, r2，递归地判断两棵子树是否对称。

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代码

/** * 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 treeSymmetric(root.left, root.right); } private boolean treeSymmetric(TreeNode r1, TreeNode r2) { if (r1 == null && r2 == null) { return true; } if ((r1 == null && r2 != null) || (r1 != null && r2 == null)) { return false; } if (r1.val != r2.val) { return false; } if (treeSymmetric(r1.left, r2.right) && treeSymmetric(r1.right, r2.left)) { return true; } else { return false; } } }