def permutations(nums):
    result = []

    def backtrack(start):
        if start == len(nums):
            result.append(nums[:])  # found a permutation
            return
        for i in range(start, len(nums)):
            nums[start], nums[i] = nums[i], nums[start]  # swap
            backtrack(start + 1)
            nums[start], nums[i] = nums[i], nums[start]  # swap back

    backtrack(0)
    return result

# [1, 2, 3] → 6 permutations