Remove Duplicate Characters in String

public class RemoveDuplicates {
    static String removeDups(String s) {
        if(s == null || s.length() <= 1) return s;
        boolean[] hit = new boolean[256];
        int len = s.length();
        String res = "";
        for( int i = 0 ; i < len ; i++) {
            if(hit[(int)s.charAt(i)]) {
                continue;
            }
            else {
                hit[(int)s.charAt(i)] = true;
                res += s.charAt(i);
            }
        }
        
        return  res;
    }

    public static void main(String[] args) {
        System.out.println(removeDups("asfass"));
        // Output: -> asf
        System.out.println(removeDups("aaaasssaa"));
        // Output: -> as
    }
}

Time Complexity: O(n)