4 kyu
link to kata
my solution
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
If the digits can't be rearranged to form a bigger number, return -1 (or nil in Swift, None in Rust):
9 ==> -1
111 ==> -1
531 ==> -1