#LeetCode# 101. Symmetric Tree

https://leetcode.com/problems/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  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.

代码：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        if(!root) return true;
        return another(root -> left, root -> right);
    }
    bool another(TreeNode* lt, TreeNode* rt) {
        if(!lt && !rt) return true;
        if(!lt || !rt) return false;
        else {
            if(lt -> val != rt -> val) return false;
            else return another(lt -> left, rt -> right) && another(lt -> right, rt -> left);
        }
    }
};

　　这个写了超级久本来想到之前的一个反转二叉树 反转之后进行比较 但是 WA 掉好几发一直没改过来