leetCode-101:Symmetric Tree

问题描述

给定一颗二叉树，要求判断二叉树是否是对称的（以根节点的垂直线进行划分）。题目链接：**点我**

样例输入输出

输入：[1,2,2]

输出：true

解释：树结构如下

1

/ \

2 2

输入：[1,2,3]

输出：false

解释：树结构如下

1

/ \

2 3

问题解法

从根节点拆分成两颗树，对于两颗树而言，要对称，则第一颗树的左节点需要对应第二颗树的右节点，第一颗树的右节点需要对应第二颗树的左节点。代码如下：

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

        return isSymmetric(root.left, root.right);
    }

    private boolean isSymmetric(TreeNode left, TreeNode right)
    {
        if (left == null && right == null)
        {
            return true;
        }
        
        if (left == null || right == null)
        {
            return false;
        }
        
        if (left.val != right.val)
        {
            return false;
        }
        
        return isSymmetric(left.left, right.right) && isSymmetric(left.right, right.left);
    }
}