LeetCode——Permutations

class Permutations:
    def perm(self, num, li, start, end):
        s = []
        if start == end:
            for i in num:
                s.append(i)
            li.append(s)
        else:
            for i in range(start, end):
                num[start], num[i] = num[i], num[start]
                self.perm(num, li, start + 1, end)
                num[start], num[i] = num[i], num[start]

    # @param num, a list of integer
    # @return a list of lists of integers

    def permute(self, num):
        li = []
        if len(num) == 0:
            return []
        if len(num) == 1:
            return [num]
        self.perm(num, li, 0, len(num))
        return li


pe = Permutations()
pe.permute([3, 2, 1])