[LeetCode101]Symmetric Tree

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：

 /**
  * Definition for a binary tree node.
  * struct TreeNode {
  *     int val;
  *     TreeNode *left;
  *     TreeNode *right;
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
 class Solution {
 public:

     bool isSymmetric(TreeNode *root) {
         if (!root) return true;
         return helper(root->left, root->right);
     }

     bool helper(TreeNode* p, TreeNode* q) {
         if (!p && !q) {
             return true;
         } else if (!p || !q) {
             return false;
         }

         if (p->val != q->val) {
             return false;
         }

         return helper(p->left,q->right) && helper(p->right, q->left);
     }
 };