Shuffle a string

shuffle.java

public static String shuffle(String string) {
        StringBuilder sb = new StringBuilder(string.length());
        double rnd;
        for (char c: string.toCharArray()) {
            rnd = Math.random();
            if (rnd < 0.34)
                sb.append(c);
            else if (rnd < 0.67)
                sb.insert(sb.length() / 2, c);
            else
                sb.insert(0, c);
        }
        return sb.toString();
    }