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:

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.

分类：Tree BFS DFS

代码：

1 /** 2  * Definition for a binary tree node. 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     13     bool isSymmetric(TreeNode *root) {14         if (!root) return true;15         return helper(root->left, root->right);16     }17 18     bool helper(TreeNode* p, TreeNode* q) {19         if (!p && !q) {20             return true;21         } else if (!p || !q) {22             return false;23         }24 25         if (p->val != q->val) {26             return false;27         }28 29         return helper(p->left,q->right) && helper(p->right, q->left); 30     }31 };