Symmetric Tree leetcode java

问题描述：

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

    1
   / 
  2   2
 /  / 
3  4 4  3

But the following is not:

    1
   / 
  2   2
      
   3    3

Note:
Bonus points if you could solve it both recursively and iteratively.

分析：判断给定的一棵二叉树是不是对称的。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

 public boolean isSymmetric(TreeNode root) { //递归
        if (root == null) return true;  
        return isMiror(root.left, root.right);  
    }
   public boolean isMiror(TreeNode n1, TreeNode n2) {  //判断两棵树是否成镜像
        if (n1 == null && n2 == null) return true;  
        if (n1 == null && n2 != null) return false;  
        if (n1 != null && n2 == null) return false;  
        if (n1.val != n2.val) return false;  
        return isMiror(n1.left, n2.right) && isMiror(n1.right, n2.left);  
    }