[LeetCode每日一题]101. Symmetric Tree

题目如下：

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:
eg:
          1
         /  \
       2     2
      / \   /  \
     3  4  4  3
        true
        
         1
        / \
       2   2
        \     \
         3     3
        false

给定一棵二叉树，让我们来判断它是不是对称的。解这个问题的思路是从根节点出发，分别向左儿子和右儿子两个方向出发遍历二叉树，先看左右儿子节点的值是否相等，不等则返回false。然后调用递归func（left.left， right.right）和func（left.right, right.left）,两者均为真时返回true，否则返回false。
代码如下：

/**
 * 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 isSymmetric(root.left, root.right);
    }

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