input: Write a function to check if a given binary tree is a subtree of another binary tree. If it is a subtree, return True, otherwise return False. If either tree is empty, return False.
  assert isSubTree(Node(1), Node(1))==True
  assert isSubTree(Node(1), Node(2))==False
  assert isSubTree(None, Node(1))==False
  assert isSubTree(Node(1), None)==False
  assert isSubTree(None, None)==False
output: def isSubTree(s, t):
      if not s or not t:
          return False
      if s.val == t.val and isSameTree(s, t):
          return True
      return isSubTree(s.left, t) or isSubTree(s.right, t)
  
  def isSameTree(s, t):
      if not s and not t:
          return True
      if not s or not t:
          return False
      return s.val == t.val and isSameTree(s.left, t.left) and isSameTree(s.right, t.right)