Why we only update the cnt array when a[i] equal to chosen digit? I may think that when the conditieon is enough when we firstly seen the digit x? Like: n = 6, a = {1, 4, 4, 3, 1, 4} with chosen digit = 3. It only update when we at index 4 (a[4] = 3), so that we have {1, 3, 4, 4}. But there are also in the index 6, we can have an bad array {1, 1, 3, 4, 4, 4}, but in this case the author not update the cnt array.

Code:

if(a[i] == x) {
  while(j <= i) {
    cnt[pref[j]]++;
    j++;
  }
}

I need solution of problem F please.