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.
  assert isSubTree(Node(4), Node(4)) == True
  assert isSubTree(Node(4), Node(5)) == False
  assert isSubTree(Node(4), None) == False
  assert isSubTree(None, Node(4)) == True
  assert isSubTree(None, None) == True
output: def isSubTree(s, t):
      if not s:
          return not t
      if not t:
          return False
      if isSameTree(s, t):
          return True
      return isSubTree(s.left, t) or isSubTree(s.right, t)
  
  def isSameTree(p, q):
      if not p and not q:
          return True
      if not p or not q:
          return False
      return p.val == q.val and isSameTree(p.left, q.left) and isSameTree(p.right, q.right)