Symmetric Tree

原题链接：https://leetcode.com/problems/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:

    1
   / 
  2   2
 /  / 
3  4 4  3

But the following [1,2,2,null,3,null,3] is not:

    1
   / 
  2   2
      
   3    3
目前，我只实现了递归版本，非递归版本稍后更新。
递归版本代码如下：
语言：C++

/**
 * 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 == NULL)    
            return true;
        else
            return isSymmetric(root->left,root->right);
    }
    bool isSymmetric(TreeNode* lefttree,TreeNode* righttree)
    {
        if((lefttree == NULL)&&(righttree == NULL)) 
        {
            return true;
        }
        else if((lefttree == NULL)&&(righttree != NULL))   
        {
            return false;
        }
        else if((lefttree != NULL)&&(righttree == NULL))   
        {
            return false;
        }
        else if((lefttree != NULL)&&(righttree != NULL)&&(lefttree->val != righttree->val))
        {
            return false;
        }
        else
        {
            return isSymmetric(lefttree->left,righttree->right) && isSymmetric(lefttree->right,righttree->left);
        }
    }
};

 此代码在最开始提交时，结果总是不对。原因：

将 "lefttreel != righttree" 改为"lefttree->val != righttree->val"，需要比较的不是节点，而是节点的value。

另外，今天get到一句感触颇深的话：不能靠Debug写代码，与各位共勉，谢谢！