input: Write a function to check if the first binary tree is a subtree of the second binary tree. If it is, return True, otherwise return False. If the first tree is empty, return True. If the second tree is empty while the first tree is not, return False.
  assert isSubTree(Node(1), None)==True
  assert isSubTree(None, Node(1))==False
  assert isSubTree(Node(1), Node(2))==False
  assert isSubTree(Node(1, Node(2), Node(3)), Node(1, Node(2), Node(3)))==True
  assert isSubTree(Node(1, Node(2), Node(3)), Node(4, Node(1, Node(2), Node(3)), Node(5)))==True
output: def isSubTree(t1, t2):
      if t1 is None:
          return True
      if t2 is None:
          return False
      
      def isSameTree(p, q):
          if p is None and q is None:
              return True
          if p is None or q is None:
              return False
          return p.val == q.val and isSameTree(p.left, q.left) and isSameTree(p.right, q.right)
      
      if isSameTree(t1, t2):
          return True
      
      return isSubTree(t1, t2.left) or isSubTree(t1, t2.right)