Method set

java/0981-time-based-key-value-store.java:9–12

(String key, String value, int timestamp)

    }

    public void set(String key, String value, int timestamp) {
        if (!map.containsKey(key)) map.put(key, new ArrayList<>());
        map.get(key).add(new Pair(value, timestamp));
    }

    public String get(String key, int timestamp) {
        if (!map.containsKey(key)) return "";