Flattens a list.

Use recursion.
Use Iterable.expand() to combine elements into a single list, calling flatten recursively for any elements that are Lists.

代码实现

List<T> flatten<T>(List<T> lst) {
  return lst.expand((e) => e is List ? flatten(e) : [e]).toList();
}

使用样例

flatten([1, [2], [[3], 4], 5]); // [1, 2, 3, 4, 5]