get the K - th letter
for ( int i = N - 1 ; i >= 0 ; -- i ) { K %= ( int ) size ; if ( K == 0 && char . IsLetter ( S [ i ] ) ) { return ( String ) " " + S [ i ] ; } if ( char . IsDigit ( S [ i ] ) ) { size = size / ( S [ i ] - '0' ) ; } else { size -= 1 ; } } return null ; }