occorrenze.c

#include <stdio.h>
#define N 26

int indexof(char c) {
    return c - 'a';
}

char valueof(int i) {
    return 'a' + i;
}

char occorrenza(char stringa[]) {
    int occorrenze[N] = { 0 };
    int max = 0, i;
    char *c;

    for (c = stringa; *c != '\0'; c++) {
         i = indexof(*c);
         if (++occorrenze[i] > occorrenze[max]) max = i;

    }
    return valueof(max);
}

void main() {
    char test[] = "brrrchefeddddo";
    putchar(occorrenza(test));
    putchar('\n');
}