[flatten_list] flatten list

flatten_list.py

# flatten([1, [2, 3, [4]], 5]) -> [1,2,3,4,5]
# flatten([1, [2,3], [[2], 5], 6]) -> [1,2,3,2,5,6]

def flatten(s):
  if not s:
    return []
  if isinstance(s[0], list):
    return flatten(s[0]) + flatten(s[1:])
  return s[:1] + flatten(s[1:])