Question

I have a program which I found online which basically tells whether the String contains all unique characters, below is the code

private static boolean areCharsUnique(String str) {
        if (str.length() > 256)
            return false;
        int checker = 0;
        for (int i = 0; i < str.length(); i++) {
            int val = str.charAt(i) - 'a';
            if ((checker & (1 << val)) > 0) {
                return false;
            }
            checker |= (1 << val);
        }
        return true;
    }

I am baffled by this line of code if ((checker & (1 << val)) > 0) and also

checker |= (1 << val);

I know that << is a left shift operator, but how exactly left shifting helps in the above situation?

In short how does the above program work?

Answer 1

This code works under the assumption that ASCII character set has consecutive character in its mapping so that a == 97, b == 98, and so on.

Starting from this you can calculate a delta distance from the a character, eg 'e' - 'a' = 5. This distance is used to set (through checker |= (1 << val)) a bit in a integer number (which has 32 bits).

So if a 'e' character is found then bit at index 5 is set to 1 for checker.

This is done for every char by ensuring that you never find a bit that has been already set previously (through if (checker & (1 << val)) > 0)).

This works only for lowercase characters (even because int has 32 bits). An HashSet<Character> would be surely better.