Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:
But the following [1,2,2,null,3,null,3] is not:

Recursively Solution:

/**
 * 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 isSame(TreeNode* t1, TreeNode* t2){
        if(NULL == t1 && NULL == t2) return true;
        else if(NULL == t1 && NULL != t2) return false;
        else if(NULL != t1 && NULL == t2) return false;
        else return t1->val == t2->val && isSame(t1->left, t2->right) && isSame(t1->right, t2->left);
    }
    bool isSymmetric(TreeNode* root) {
        if(NULL == root) return true;
        return isSame(root->left, root->right);
    }
};