【leetcode】101. Symmetric Tree

题目要求：

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

给一颗二叉树，判断它是不是中心对称的

思路：如果左右子树都不为空，就看左子树的左子树的值和右子树的右子树的值，以及左子树的右子树的值和右子树的左子树的值是否相等

/** * 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 isEqual(root.left,root.right);    }     //辅助函数，判断传进来的两个子树是否相等    public boolean isEqual(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 isEqual(left.right,right.left)&&isEqual(left.left,right.right);    }}