python - 在 python 中展开列表

Question

问题:

Write a function named "unflatten" which takes a list as an argument and constructs a nested list.

The format of the argument list is as follows:

An integer item indicates a start of a nested list Non-integer items will be the content of the nested list For instance,

[2, 'a', 3, 'b', 'c', 'd'] is converted to ['a', ['b', 'c', 'd']] The first number, 2, indicates that the upper list will contain 2 items. 'a' is the first item of this upper list. The number 3 indicates a start of a new sub-list which contains 3 items.

示例运行:

>>> unflatten([2, 'x', 'y'])
['x', 'y']
>>> unflatten([ 3, "a", "b", 3, "t", "y", "u" ])
['a', 'b', ['t', 'y', 'u']]
>>> unflatten([ 4, "a", "b", 3, "c", "d", 2, "x", "y", 2, "w" , 3, "t", "y", "u" ])
['a', 'b', ['c', 'd', ['x', 'y']], ['w', ['t', 'y', 'u']]]

我做了一个简单的递归。这是我的代码:

def unflatten(LIST):
    if not len(LIST):
       return []
    elif isinstance(LIST[0], int):
        return [unflatten(LIST[1:])]
    else:
        return [LIST[0]]  + unflatten(LIST[1:])


>>> unflatten([ 4, "a", "b", 3, "c", "d", 2, "x", "y", 2, "w" , 3, "t", "y", "u" ])
[['a', 'b', ['c', 'd', ['x', 'y', ['w', ['t', 'y', 'u']]]]]]

现在如您所见，列表的长度在我的基本递归中不受控制，因此它只是在末尾结束所有列表。

我不知道如何递归或迭代地跟踪长度。如果您建议一种无需导入任何模块即可执行此操作的方法，我会很高兴。

Answer 1

跟踪位置的一种方法是返回它。在下面的代码中，我使用一个辅助函数，该函数返回部分构建的未展平列表以及展平列表中的当前索引。

def unflatten(l):
  def helper(l, start):
    if isinstance(l[start], int):
      ret = []
      start += 1
      for _ in range(l[start - 1]):
        sub, start = helper(l, start)
        ret.append(sub)
      return ret, start
    else:
      return l[start], start + 1
  return helper(l, 0)[0]

print unflatten([2, 'x', 'y'])
print unflatten([ 3, "a", "b", 3, "t", "y", "u" ])
print unflatten([ 4, "a", "b", 3, "c", "d", 2, "x", "y", 2, "w" , 3, "t", "y", "u" ])