Designing function f(f(n)) == -n

Question

A question I got on my last interview:

Design a function f, such that:

f(f(n)) == -n

Where n<

Answer 1

works for n= [0 .. 2^31-1]

int f(int n) {
  if (n & (1 << 31)) // highest bit set?
    return -(n & ~(1 << 31)); // return negative of original n
  else
    return n | (1 << 31); // return n with highest bit set
}