[Leetcode] 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.

Analysis:

class Solution {
public:

    bool helper(TreeNode *left, TreeNode* right) {
        if (left == NULL && right == NULL)
            return true;
        else if (left == NULL || right == NULL)
            return false;
        else
        {
            return left->val == right->val 
            && helper(left->left, right->right)
            && helper(left->right, right->left);
        }
    }
    bool isSymmetric(TreeNode *root) {
        if (root == NULL)
            return true;
        else
            return helper(root->left, root->right);
    }
};