import collections class OrderedSet(collections.MutableSet): def __init__(self, iterable=None): self.end = end = [] end += [None, end, end] # sentinel node for doubly linked list self.map = {} # key --> [key, prev, next] if iterable is not None: self |= iterable def __len__(self): return len(self.map) def __contains__(self, key): return key in self.map def add(self, key): if key not in self.map: end = self.end curr = end[1] curr[2] = end[1] = self.map[key] = [key, curr, end] def discard(self, key): if key in self.map: key, prev, next = self.map.pop(key) prev[2] = next next[1] = prev def __iter__(self): end = self.end curr = end[2] while curr is not end: yield curr[0] curr = curr[2] def __reversed__(self): end = self.end curr = end[1] while curr is not end: yield curr[0] curr = curr[1] def pop(self, last=True): if not self: raise KeyError('set is empty') key = self.end[1][0] if last else self.end[2][0] self.discard(key) return key def __repr__(self): if not self: return '%s()' % (self.__class__.__name__,) return '%s(%r)' % (self.__class__.__name__, list(self)) def __eq__(self, other): if isinstance(other, OrderedSet): return len(self) == len(other) and list(self) == list(other) return set(self) == set(other) if __name__ == '__main__': s = OrderedSet('abracadaba') t = OrderedSet('simsalabim') print(s | t) print(s & t) print(s - t)