// put integer array into random order
//
void shuffle( int a[], int n)
{
  int t, i;

  while( n > 1)
  {
    i = rand() % n;  // random position 0 ... n-1
    --n;
    t = a[i]; a[i] = a[n]; a[n] = t; // swap a[i] and a[n]
  }
}