get the K - th letter
for ( var i = N - 1 ; i >= 0 ; -- i ) { K %= size ; if ( K == 0 && isAlphaNumeric ( S [ i ] ) ) return " " + S [ i ] ; if ( Number . isInteger ( parseInt ( S [ i ] ) ) ) size = size / ( S [ i ] . charCodeAt ( 0 ) - ' ' . charCodeAt ( 0 ) ) ; else size -= 1 ; } }