leetcode101 Symmetric Tree

101 Symmetric Tree

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

解法

递归方法，分析左右两子树是否对称。

/** * 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 isSymmetricHelper(root.left, root.right);    }    public boolean isSymmetricHelper(TreeNode left, TreeNode right) {        if (left == null && right == null) {            return true;        }        if (left == null || right == null) {            return false;        }        if (left.val == right.val) {            return isSymmetricHelper(left.left, right.right) && isSymmetricHelper(left.right, right.left);        }        return false;    }}