问题描述
我最近遇到了使用以下两种方法实现数据结构的挑战：
set(key,value,time)：在指定时间设置键值。get(key,time)：在指定时间或更早的时间获取key的值。
如果我们在特定时间设置一个键，它将永远保持该值或直到它在以后设置为止。
示例
示例 1
d.set(1,1,0) // set key 1 to value 1 at time 0
d.set(1,2,2) // set key 1 to value 2 at time 2
d.get(1,1) // get key 1 at time 1 should be 1
d.get(1,3) // get key 1 at time 3 should be 2
示例 2
d.set(1,5) // set key 1 to value 1 at time 5
d.get(1,0) // get key 1 at time 0 should be null
d.get(1,10) // get key 1 at time 10 should be 1
示例 3
d.set(1,0) // set key 1 to value 2 at time 0
d.get(1,0) // get key 1 at time 0 should be 2
解决方法
这是我的实现：
import java.util.HashMap;
import java.util.Map;
import java.util.NavigableMap;
import java.util.TreeMap;
/**
* An Implementation a Map with the notion of Time.
*
* @param <K> the type of keys maintained by this map
* @param <V> the type of mapped values
* @param <T> the type of time tracked by this map.
* T MUST implement {@link Comparable} to have a notion of comparing times.
* @author Hari Krishnan
* @see Map
* @see NavigableMap
*/
public class TimedMap<K,V,T extends Comparable<T>> {
private final Map<K,NavigableMap<T,V>> map = new HashMap<>();
/**
* Associates the specified value with the specified key at the specified time in this map.
* If the map previously contained a mapping for the key at this time,* the old value is replaced by the specified value.
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @param time time at which said association should occur
*/
public void set(K key,V value,T time) {
if (!map.containsKey(key))
map.put(key,new TreeMap<>());
map.get(key).put(time,value);
}
/**
* Returns the value to which the specified key is mapped at the specified time or earlier,* or {@code null} if this map contains no mapping for the key at the specified time or earlier.
*
* @param key the key whose associated value is to be returned
* @param time the time(or earlier) at which the values associated with the key is to be returned.
* @return The value to which the specified key is mapped at the specified time or earlier,or
* {@code null} if this map contains no mapping for the key at the specified time or earlier.
*/
public V get(K key,T time) {
try {
return map.get(key).floorEntry(time).getValue();
} catch (NullPointerException ex) {
return null;
}
}
}
欢迎改进和建议！