[leetcode.com]算法题目 - Symmetric Tree

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.

 1 /**

 2  * Definition for binary tree

 3  * struct TreeNode {

 4  *     int val;

 5  *     TreeNode *left;

 6  *     TreeNode *right;

 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 8  * };

 9  */

10 class Solution {

11 public:

12     bool isSymmetric(TreeNode *root) {

13         // Start typing your C/C++ solution below

14         // DO NOT write int main() function

15         if (!root) return true;

16         

17         if(!root->left && !root->right)

18             return true;

19             

20         if(!root->left && root->right)

21             return false;

22         

23         if(root->left && !root->right)

24             return false;

25             

26         if(root->left->val == root->right->val)

27             return symmetricTree(root->left, root->right);

28         else

29             return false;

30     }

31     

32     bool symmetricTree(TreeNode *a, TreeNode *b){

33         if(!a && !b)

34             return true;

35             

36         if((!a && b) || (a && !b))

37             return false;

38             

39         if(a->val == b->val)

40             return symmetricTree(a->left, b->right) && symmetricTree(a->right, b->left);

41         else

42             return false;

43     }

44 };

View Code

思路：和判断两个tree是否相同有点类似，但是要注意这里判断的是是否对称。好久不写代码，看了别人写的这道题答案，感觉自己写的代码很不标准。不管怎样，先贴出来，慢慢进步吧。