foo(foo(x)) = -x

Question

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Interview question: f(f(n)) == -n

Like I wrote above, implement a function whose prototype is

int foo(int n);

such that foo(foo(x)) returns -x

Answer 1

int foo(int n)
{
    static int called = 1;
    if (called == 2)
    {
        called = 1;
        return n;
    }
    else
    {
        called = 2;
        return -n;
    }
}