Write permutations(nums) that returns every ordering of nums as a list of lists.
- The values are distinct integers;
1 <= len(nums) <= 7(so up to 5,040 results). - The permutations can come in any order, but each must appear exactly once.
- Don't use
itertools.permutations; build them yourself.
permutations([4, 9]) # [[4, 9], [9, 4]] in some order
permutations([1, 2, 3]) # 6 lists: [1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], [3,2,1]
Show hint
build an ordering one position at a time, remembering which indices are already used; after exploring a choice, undo it before trying the next.