Create a function that takes a positive integer and returns the next bigger number that can be formed by rearranging its digits. For example:
12 ==> 21
513 ==> 531
2017 ==> 2071
nextBigger(num: 12) // returns 21
nextBigger(num: 513) // returns 531
nextBigger(num: 2017) // returns 2071
If the digits can't be rearranged to form a bigger number, return -1 (or nil in Swift):
9 ==> -1
111 ==> -1
531 ==> -1
nextBigger(num: 9) // returns nil
nextBigger(num: 111) // returns nil
nextBigger(num: 531) // returns nil