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 [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.

class Solution:
    def isSymmetric(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        def com(left,right):
            if left is None and right is None:
                return True
            if left is None or right is None:
                return False
            if left.val!=right.val:
                return False
            return com(left.left,right.right) and com(left.right,right.left)
        if not root:
            return True
        return com(root.left,root.right)