LeetCode-101:Symmetric Tree

原题描述如下:

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

题意：判断一棵二叉树是否为对称二叉树。

解题思路：递归求解，注意代码鲁棒性。

Java代码：

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