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题目：（困难）

标签：字符串、栈

解法一（构造集合类）：

class Expression(set):    def __add__(self, other):        ans = Expression()        for elem in self:            ans.add(elem)        for elem in other:            ans.add(elem)        return ans    def __mul__(self, other):        ans = Expression()        for elem1 in self:            for elem2 in other:                ans.add(elem1 + elem2)        return ansclass Solution:    def braceExpansionII(self, expression: str) -> List[str]:        stack = [[Expression()]]        for ch in expression:            if ch.isalpha():                if not stack[-1][-1]:                    stack[-1][-1] = Expression((ch,))                else:                    stack[-1][-1] = stack[-1][-1] * Expression((ch,))            else:                if ch == "{":                    stack.append([Expression()])                elif ch == ",":                    stack[-1].append(Expression())                elif ch == "}":                    now_exp = Expression()                    for exp in stack.pop():                        now_exp = now_exp + exp                    if not stack[-1][-1]:                        stack[-1][-1] = now_exp                    else:                        stack[-1][-1] = stack[-1][-1] * now_exp            # print(ch, "->", stack)        return sorted(list(stack[0][0]))

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