int n_used_chars = 0;
int chars[256], loc_of_char[256];
for (int i = 0; i < 256; i++) used_chars[i] = loc_of_char[i] = i;

for (int i = 0; i < string_length; i++) {
  char c = string[i];
  int loc = loc_of_char[c];
  if (loc >= n_used_chars) {
    // Character c has not been used before. Swap it down to the used set.
    chars[loc] = chars[n_used_chars];
    loc_of_char[chars[loc]] = loc;
    chars[n_used_chars] = c;
    loc_of_chars[c] = n_used_chars++;
  }
}
// At this point chars[0..n_used_chars - 1] contains all the used chars.
// and chars[n_used_chars..255] contains the unused ones!