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public class TimeMap {
    private Map<String, TreeMap<Integer, String>> m;

    public TimeMap() {
        m = new HashMap<>();
    }

    public void set(String key, String value, int timestamp) {
        m.computeIfAbsent(key, k -> new TreeMap<>()).put(timestamp, value);
    }

    public String get(String key, int timestamp) {
        if (!m.containsKey(key)) return "";
        TreeMap<Integer, String> timestamps = m.get(key);
        Map.Entry<Integer, String> entry = timestamps.floorEntry(timestamp);
        return entry == null ? "" : entry.getValue();
    }
}