gistfile1.c

for (; *s; s++) t[*s]++;

void count(char *s, int t1, int t2, …, int t255)
{
    if (*s == '\0') {
        // output answer
    } else if (*s == 1) {
        count(str + 1, t1 + 1, t2, …, t255); 
    } else if (*s == 2) {
        count(str + 1, t1, t2 + 1, …, t255);
    } else if (*s == 3) {
    // … omitted …
}

void derive_copy(int *origin, *int derived, int idx, int size, int pos, int value)
{
    if (idx == size) return;
    derived[idx] = (idx == pos) ? value : origin[idx];
    derive-copy(origin, derived, idx + 1, size, pos, value);
}

int *derive(int *origin, int size, int pos, int value)
{
    int *derived = (int *)malloc(sizeof(int) * size);
    derive_copy(origin, derived, 0, size, pos, value);
    return derived;
}

void count(char *s, int *t)
{
    if (*s == '\0') {
        // output answer
    } else {
        count(str + 1, derived(t, 256, *s, t[*s] + 1));
    }
}