search through all the indiced at which the current character occurs . For each index greater than prev , take the index and move to the next position , and add to the answer .
int answer = 0 ; for ( int i = 0 ; i < index . size ( ) ; i ++ ) { if ( index . get ( i ) . compareTo ( prev ) >= 0 ) { answer = ( answer % mod + calculate ( pos + 1 , index . get ( i ) , s , index ) % mod ) % mod ; } }