this should work for C as the problem says: 1 <= ar[i] <=100

int i, res = 0;
int map[101] = {0};

for(i = 0; i < n; i++) {
    map[ar[i]]++;
    if (map[ar[i]] == 2) {
        res++;
        map[ar[i]] = 0;
    }
}
return res;