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