Mark those characters which are not present in both lowercase and uppercase
let mp = new Map ( ) ; for ( i = 0 ; i < 26 ; i ++ ) { if ( small [ i ] != 0 && caps [ i ] == 0 ) { mp . set ( String . fromCharCode ( i + 97 ) , 1 ) ; } else if ( caps [ i ] != 0 && small [ i ] == 0 ) mp . set ( String . fromCharCode ( i + 65 ) , 1 ) ; }