numSplits solution without arrays

count.c

#include <stdio.h>

#define hash(c) 1 << (c - 'a')

int numSplits(char * s) {
  unsigned int n = 0, l = 0, r = 0, i = 0, j = 0, cur = 0;

  while (s[i]) {
    r |= hash(s[i++]);
  }

  i = 0;

  while (s[++i]) {
    cur = hash(s[i-1]);
    l |= cur;
    r &= ~cur;
    j = i;
    while(s[j]) {
      if (cur & hash(s[j++])) {
        r |= cur;
        break;
      }
    }
    n += __builtin_popcount(l) == __builtin_popcount(r);
  }
  return n;
}

int main(int argc, char ** argv) {
  if (argc != 2) return -1;
  printf("%i\n", numSplits(argv[1]));
}

smallcount.c

#include <stdio.h>

size_t hash(char * s, size_t l) {
  size_t h = 0, b = 0, i = 0;
  while (s[i] && (i < l || l < 0)) {
    b |= (1 << (s[i++] - 'a'));
  }
  while (b) {
    h += b & 1;
    b >>= 1;
  }
  return h;
}

int numSplits(char * s){
  if (!*s) return 0;
  int n = 0;
  for (size_t i = 1; s[i]; ++i) {
    n += hash(s, i) == hash(s+i, -1);
  }
  return n;
}

int main(int argc, char ** argv) {
  if (argc != 2) return -1;
  printf("%i\n", numSplits(argv[1]));
}