Remove duplicate chars from a string for english word. Complexity = O (n)

gistfile1.java

public static String removeDuplicateChars2(String s)
{
    if (s == null) {
    	return s;
    }
    int len = s.length();
    if (len < 2){
	return s;
    }

    char[] c = s.toCharArray();

    boolean[]hit = new boolean[256];
    for (int i = 0; i < hit.length; i++) {
	hit[i] = false;
    }

    hit[c[0]] = true;
    int k = 1;
    for (int i = 1; i < len; i++) {
	if (!hit[c[i]]) {
	    hit[c[i]] = true;
	    c[k] = c[i];
	    k++;
	}
    }

    return new String(c);
}