input: Write a python function to check if a binary tree is a subtree of another binary tree.
  assert isSubTree(T1, S1) == True
  assert isSubTree(T2, S2) == False
  assert isSubTree(T3, S3) == True
output: def isSubTree(T, S):
      def areIdentical(root1, root2):
          if root1 is None and root2 is None:
              return True
          if root1 is None or root2 is None:
              return False
          return (root1.val == root2.val and 
                  areIdentical(root1.left, root2.left) and 
                  areIdentical(root1.right, root2.right))
      
      if S is None:
          return True
      if T is None:
          return False
      if areIdentical(T, S):
          return True
      return isSubTree(T.left, S) or isSubTree(T.right, S)