Java: Print a unique character in a string

Question

I\'m writing a program that will print the unique character in a string (entered through a scanner). I\'ve created a method that tries to accomplish this but I keep getting

Answer 1

This String algorithm is used to print unique characters in a string.It runs in O(n) runtime where n is the length of the string.It supports ASCII characters only.

static String printUniqChar(String s) {
    StringBuilder buildUniq = new StringBuilder();
    boolean[] uniqCheck = new boolean[128];
    for (int i = 0; i < s.length(); i++) {
        if (!uniqCheck[s.charAt(i)]) {
            uniqCheck[s.charAt(i)] = true;
            if (uniqCheck[s.charAt(i)])
                buildUniq.append(s.charAt(i));
        }
    }