leetcode 第101题 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.

confused what “{1,#,2,3}” means? > read more on how binary tree is serialized on OJ.

OJ’s Binary Tree Serialization:

The serialization of a binary tree follows a level order traversal, where ‘#’ signifies a path terminator where no node exists below.

Here’s an example:

1/ \

2 3

/

4

\

5

The above binary tree is serialized as “{1,2,3,#,#,4,#,#,5}”.

思路：递归实现，自顶向下比较。

c++实现：

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
bool checknode(TreeNode *leftnode,TreeNode *rightnode){
if(leftnode == NULL && rightnode == NULL)
return true;
if(leftnode == NULL || rightnode == NULL)
return false;
return leftnode->val == rightnode->val && checknode(leftnode->left,rightnode->right) && checknode(leftnode->right,rightnode->left);
}
bool isSymmetric(TreeNode *root) {
if(root == NULL)
return true;
return checknode(root->left,root->right);
}
};