int is_sorted(const int *a, int n);
// REQUIRES: n >= 1. Elements a[0] ... a[n-1] exist.
// PROMISES
//   Return value is  1 if n == 1, or if n > 1 and all of
//   a[0] <= a[1] ... a[n-2] <= a[n-1] are true.
//   Otherwise, return value is 0.

int all_different(const int *a, int n);
// REQUIRES: n > 1. Elements a[0] ... a[n-1] exist.
// PROMISES: Return value is 1 if no two elements among
// a[0] ... a[n-1] are equal to each other.
//   Otherwise, return value is 0.

int is_alternating(const int *a, int n);
// Decides whether array elements form an alternating sequence.  In an
// alternating sequence, each number has the opposite sign of the
// previous number in the sequence.  Zero is neither positive nor negative,
// so any sequence that includes 0 is not an alternating sqeuence
//
// Examples: {-1, 1, -1, 1} and {1, -1, 2, -3, 5} are alternating sequences,
// but {-2, 3, 4, -5} and {10, -20, 0, -30} are not.
//
// REQUIRES: n >= 1. Elements a[0] ... a[n-1] exist.
// PROMISES
//    The return value is 1 if n == 1.
//    If n > 1, the return value is 1 if a[0] ... a[n-1] form
//    an alternating sequence.
// PROMISES
//    Otherwise, the return value is 0.