CareerCup

public static String removeRedundantChars(char[] input){
        //assume ASCII characters only 
        boolean[] charTable = new boolean[128];
        int i = 0;
        for(int j = 0; j < input.length; j++){
            if (!charTable[input[j]])
            {
                charTable[input[j]] = true;
                input[i] = input[j];
                i++;
            }
        }
        return new String(input, 0, i);
    }

This solution takes O(1) space - BitSet and result StringBuilder:

public class RedundantCharacters {

    public static void main(String[] args) {
        new RedundantCharacters().solve();
    }

    public void solve() {
        String s = "qwertyasdfghytrewq";

        System.out.print(removeRedundant(s));
    }

    public String removeRedundant(String s) {
        StringBuilder sb = new StringBuilder();

        BitSet bitSet = new BitSet();

        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            if (!bitSet.get(ch)) {
                sb.append(ch);
            }

            bitSet.set(ch);
        }

        return sb.toString();
    }
}