[leetcode.com]算法题目

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

For example, this binary tree is symmetric:

But the following is not:

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。