input: Write a python function to check if two binary trees are identical.
  assert isIdentical(tree1, tree2) == True
  assert isIdentical(tree1, tree3) == False
  assert isIdentical(None, None) == True
output: def isIdentical(tree1, tree2):
      if tree1 is None and tree2 is None:
          return True
      if tree1 is not None and tree2 is not None:
          return (tree1.val == tree2.val) and isIdentical(tree1.left, tree2.left) and isIdentical(tree1.right, tree2.right)
      return False