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);    }};