[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); 
    }
};