Return string of all 0 's  of length A
return string ( A , '0' ) ; } if ( K <= dp [ A - 1 ] [ B ] ) { return "0" + KthString ( A - 1 , B , K , dp ) ; } else { return "1" + KthString ( A , B - 1 , K - dp [ A - 1 ] [ B ] , dp ) ; } }