
find a big nonexist prime number ( magic number )

go through the list, if the number is dividable by the magic number, it is the solution.

other wise, multiply the number by the magic number

scan again, un touch the array.

Space complexity O(1)

Time Complexity O(N)
See code:
https://github.com/zyzyis/OneSizeDoesFitAll/blob/master/src/FindTheDuplicateNumber.c