题目描述

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
给定二叉树，检查它是否是自身的镜像（即，围绕其中心对称）。

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

Note:
Bonus points if you could solve it both recursively and iteratively.
如果您可以递归和迭代地解决它，那么奖励积分。

代码

递归

class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return true;
        return isMirrorTree(root.left, root.right);
    }
    
    boolean isMirrorTree(TreeNode p, TreeNode q) {
        if (p == null && q == null) return true;
        if (p == null || q == null) return false;
        if (p.val != q.val) return false;
        return isMirrorTree(p.left, q.right) && isMirrorTree(p.right, q.left);
    }
}