Leetcode #101 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

/**
 * 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 symmetric(TreeNode *p, TreeNode *q)
    {
    	if(!p && !q) return true;
    	if(p && !q || !p && q) return false;
    	if(p->val != q->val) return false;
    	bool b1 = symmetric(p->left , q->right);
    	bool b2 = symmetric(p->right , q->left);
    	return b1 && b2;
    }
    
    bool isSymmetric(TreeNode* root) {
    
    	if(!root) 
    		return true;
    	return symmetric(root,root);
    }
};