Symmetric Tree <LeetCode>

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:

    1
   / 
  2   2
      
   3    3


思路：首先把想到用递归分别遍历左右子树，不同的时，遍历左子树时，先遍历根节点，然后是左孩子，最后右孩子；遍历右子树时，县遍历根节点，然后是右孩子，最后是左孩子，每遍历到一个节点进行比较，是否相同，代码如下：//没有优化

 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         if(root==NULL)  return true;
14         TreeNode *rootl;
15         TreeNode *rootr;
16         rootl=root->left;
17         rootr=root->right;
18         return isSame(rootl,rootr);
19     }
20     
21     bool isSame(TreeNode *rootl,TreeNode *rootr)
22     {
23         if(rootl==NULL||rootr==NULL)
24         {
25             if(rootl!=NULL||rootr!=NULL)
26             {
27                 return false;
28             }
29             else return true;
30         }
31         
32         if(rootl->val!=rootr->val)  return false;
33         else
34         {
35             if(isSame(rootl->left,rootr->right))
36             {
37                 if(isSame(rootl->right,rootr->left))
38                 {
39                     return true;
40                 }
41                 else return false;
42             }
43             else return false;
44         }
45         
46     }
47 };