[算法题] 全排列

leetcode 46. 全排列

描述：
给定一个没有重复数字的序列，返回其所有可能的全排列。

示例:
输入: [1,2,3]
输出:

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

---

class Solution:
    def permute(self, nums: List[int]) -> List[List[int]]:
        
        # def backtrack(rest_nums=nums, now_seq=[]):
        #     """Version 1"""
        #     if len(rest_nums) == 0:
        #         res.append(now_seq)
        #         return
        #     for idx in range(len(rest_nums)):
        #         backtrack(rest_nums[:idx]+rest_nums[idx+1:], now_seq+[rest_nums[idx]])
        #     return
        # res = []
        # backtrack()
        # return res
        
        # def backtrack(now_len=0, now_seq=[]):
        #     """Version 2"""
        #     if now_len == N:
        #         res.append(now_seq)
        #     for i in range(N):
        #         if visited[i] == 1:
        #             continue
        #         visited[i] = 1
        #         backtrack(now_len+1, now_seq+[nums[i]])
        #         visited[i] = 0
        #     return
        # N = len(nums)
        # visited = [0]*N
        # res = []
        # backtrack()
        # return res
        
        # """Version Cheat"""
        # return list(itertools.permutations(nums))
        
        def swap(i,j,this=nums):
            this[i],this[j] = this[j],this[i]
        def backtrack(pos=0):
            """Version 3"""
            if pos == N:
                res.append(nums[:])
            for i in range(pos, N):
                swap(pos,i)
                backtrack(pos+1)
                swap(pos,i)
        N = len(nums)
        res = []
        backtrack()
        return res