sowpods.c

#include <stdio.h>
#include <string.h>

static const int NUM_LETRAS = ('Z' - 'A') + 1;

int main(int argc, char const *argv[]){
  int arregloletras[NUM_LETRAS];
  int prev=0, current=0;

  memset(arregloletras, 0, sizeof arregloletras);

  while ( (current = getchar()) != EOF ){
    if (prev && (current == prev)) {
      arregloletras[current-'A'] = 1;
    }
    prev = current;
  }

  for (int i=0; i < NUM_LETRAS; ++i) {
    if ((arregloletras[i] == 0)){
      putchar(i+'A');
    }
  }

  putchar('\n');

  return 0;
}