参考代码:

class Solution:
    def stringPermutation(self, str):
        result = []
        if str == '':
            return ['']
        s = list(str)
        s.sort()
        while True:
            result.append(''.join(s))
            s = self.nextPer(s)
            if s is None:
                break
        return result

    def nextPer(self, num):
        n = len(num)
        i = n - 1
        while i >= 1 and num[i - 1] >= num[i]:
            i -= 1
        if i == 0:
            return None
        j = n - 1
        while j >= 0 and num[j] <= num[i - 1]:
            j -= 1
        num[i - 1], num[j] = num[j], num[i - 1]
        num[i:1] = num[i:][::-1]
        return num