[lintcode] problem 659 – encode and decode strings

public String (List<String> strs) {
    StringBuilder sb = new StringBuilder();

    for (String str : strs) {
        for (char ch : str.toCharArray()) {
            if (ch == ':')
                sb.append("::");
            else
                sb.append(ch);
        }

        sb.append(":;");
    }

    return sb.toString();
}

public List<String> decode(String str) {
    List<String> result = new ArrayList<>();
    StringBuilder sb = new StringBuilder();

    for (int i = 0; i < str.length();) {
        char ch = str.charAt(i);

        if (ch == ':') {
            result.add(sb.toString());

            if (str.charAt(i + 1) != ';')
                result.add(":");

            sb.setLength(0);
            i += 2;
        }
        else {
            sb.append(ch);
            i++;
        }
    }

    return result;
}