[LeetCode] 46. Permutations_Medium tag: DFS, backtracking

 

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

Example:

Input: [1,2,3]
Output:
[
  [1,2,3],
  [1,3,2],
  [2,1,3],
  [2,3,1],
  [3,1,2],
  [3,2,1]
]

这个题目就是用for loop取nums的值，然后再recursive求去掉num之后的nums的permutation，再将两者组合 起来。

时间复杂度 = 方案总数 * 构造方案所需时间

O( n ! * n)

Code

class solution:
    def permutations(self, nums):
        ans = []
        def helper(ans, temp, nums):
            if not nums:
                ans.append(temp)
　　　　     for i in range(len(nums)):
                helper(ans, temp + [nums[i]], nums[:i] + nums[i + 1:])
        helper(ans, [], nums)
        return ans

 

利用python的itertools

class Solution:
    def permutaions(self, nums):
        return itertools.permutations(nums)