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:

    1
   / \
  2   2
 / \ / \
3  4 4  3

 

But the following [1,2,2,null,3,null,3] is not:

    1
   / \
  2   2
   \   \
   3    3

 

Note:
Bonus points if you could solve it both recursively and iteratively.

 1 class Solution {
 2 public:
 3     bool thesame(TreeNode*l,TreeNode*r){
 4         if(l==NULL&&r==NULL)return true;
 5         else if(l==NULL||r==NULL)return false;
 6         bool same=true;
 7         same&=(l->val==r->val);
 8         if(!same)return false;
 9         same&=thesame(l->left,r->right);
10         if(!same)return false;
11         same&=thesame(l->right,r->left);
12         return same;
13     }
14     bool isSymmetric(TreeNode* root) {
15         if(root==NULL)return true;
16         return thesame(root->left,root->right);
17     }
18 };

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