1.1 Implement an algorithm to determine if a string has all unique characters. What if you cannot use additional data structures?

1.1 uniqueChar

public boolean uniqueChar(String str) {
    if (str == null) {
        throw new IllegalArgumentException();
    }

    int len = str.length();

    if (len == 0) {
        return true;
    }

    // There must exist at least 1 duplicated char if there are more than 256 chars.
    if (len > 256) {
        return false;
    }

    // ASCII is usually represented by 8 bits, but there are 128 different chars in total.
    boolean[] flag = new boolean[256];

    for (int i = 0; i < str.length(); i++) {
        int index = str.charAt(i);
        if (flag[index] == true) {
            return false;
        }
        else {
            flag[index] = true;
        }
    }

    return true;
}