机试刷题_字符串的排列【python】

题目：字符串的排列

from os import dup
#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 
# @param str string字符串 
# @return string字符串一维数组
#
class Solution:
    def backtrack(self,res,state,choices,selected):
    	# 当状态长度等于元素数量时，记录解
        if len(state)==len(choices):
            res.append(state)
            return
        dupilcate = set()
        for i in range(len(choices)):
        	# 剪枝：不允许重复选择元素，且不允许重复选择相等元素
            if not selected[i] and choices[i] not in dupilcate:
            	 #记录选择过的值
                dupilcate.add(choices[i])
                selected[i] = True
                state  += choices[i]
                # 进行下一轮选择
                self.backtrack(res,state,choices,selected)
                # 回退：撤销选择，恢复到之前的状态
                selected[i] = False
                state = state[:-1]


    def Permutation(self , str: str) -> List[str]:
        res = []
        if len(str)==0:
            return res
        tmp = "".join(sorted(list(str)))
        self.backtrack(res,state ="",choices = tmp,selected=[False]*len(tmp))
        return res