46. Permutations.py

class Solution(object):
    def permute(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        ans = []

        def backtrack(first):
            if first == len(nums):
                ans.append(nums[:])
            for i in range(first, len(nums)):
                nums[first], nums[i] = nums[i], nums[first]
                backtrack(first + 1)
                nums[first], nums[i] = nums[i], nums[first]

        backtrack(0)
        return ans