input: Write a function to check if one binary tree is a subtree of another binary tree. If the first tree is a subtree of the second tree, return True; otherwise, return False. If the second tree is empty or if both trees are empty, return False.
  assert isSubTree(Node(1), Node(1))==True
  assert isSubTree(Node(1), Node(2))==False
  assert isSubTree(Node(1), None)==False
  assert isSubTree(None, None)==False
output: def isSubTree(s, t):
      if not t:
          return False
      if not s:
          return False
      
      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)
      
      if isSameTree(s, t):
          return True
      return isSubTree(s, t.left) or isSubTree(s, t.right)