46. Permutations 排列

 Given a collection of distinct numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:

[
  [1,2,3],
  [1,3,2],
  [2,1,3],
  [2,3,1],
  [3,1,2],
  [3,2,1]
]

class Solution:
    def permute(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        res = []
        s = set(nums)
        def gen(l, added):
            if len(l) is len(nums):
                res.append(l[:])
            else:
                for i in s:
                    if not i in added:
                        l.append(i)
                        added.add(i)
                        gen(l, added)
                        l.pop()
                        added.remove(i)
        gen([], set())
        return res

来自为知笔记(Wiz)