Every Ordering

nums holds distinct integers. Return every way to line them all up. Each line-up uses every number once. A different order is a different line-up, so [4, 6] and [6, 4] are both in the answer.

• Return an array of arrays. Each inner array is as long as nums.
• List each line-up once only.
• The line-ups can come in any order in the result. But the order of numbers inside a line-up is what makes it that line-up, so that part matters.
• nums has 0 to 7 numbers, each between -10 and 10. If it is empty, return [[]], because there is exactly one way to line up nothing.
• Don't change nums.