Initializing vector to store count of strings .
Vector < Integer > counts = new Vector < Integer > ( ) ; for ( int i = 0 ; i < 5 ; i ++ ) { counts . add ( 1 ) ; } for ( int i = 2 ; i <= N ; i ++ ) { for ( int j = 3 ; j >= 0 ; j -- ) counts . set ( j , counts . get ( j ) + counts . get ( j + 1 ) ) ; } int ans = 0 ;