how space complexity of this code is O(n)?

Question

    public static boolean isUniqueChars2(String str) {
        boolean[] char_set = new boolean[256];

        for (int i = 0; i < str.length(); i++) {
            int val = str.charAt(i);
            if (char_set[val])
                return false;

            char_set[val] = true;
        }

        return true;
    }

Answer 1

The size of your input is obviously O(n), but the memory requirements of this function is O(1), since the array has constant size. The time complexity is O(n) though, since it iterates over the string..