PattySung

Design a time-based key-value data structure that can store multiple values for the same key at different time stamps and retrieve the key’s value at a certain timestamp.

Implement the TimeMap class:

TimeMap() Initializes the object of the data structure. void set(String key, String value, int timestamp) Stores the key key with the value value at the given time timestamp. String get(String key, int timestamp) Returns a value such that set was called previously, with timestamp_prev <= timestamp. If there are multiple such values, it returns the value associated with the largest timestamp_prev. If there are no values, it returns “”.

Example 1:

Input [“TimeMap”, “set”, “get”, “get”, “set”, “get”, “get”] [[], [“foo”, “bar”, 1], [“foo”, 1], [“foo”, 3], [“foo”, “bar2”, 4], [“foo”, 4], [“foo”, 5]] Output [null, null, “bar”, “bar”, null, “bar2”, “bar2”]

Explanation TimeMap timeMap = new TimeMap(); timeMap.set(“foo”, “bar”, 1); // store the key “foo” and value “bar” along with timestamp = 1. timeMap.get(“foo”, 1); // return “bar” timeMap.get(“foo”, 3); // return “bar”, since there is no value corresponding to foo at timestamp 3 and timestamp 2, then the only value is at timestamp 1 is “bar”. timeMap.set(“foo”, “bar2”, 4); // store the key “foo” and value “bar2” along with timestamp = 4. timeMap.get(“foo”, 4); // return “bar2” timeMap.get(“foo”, 5); // return “bar2”

<解題>

class TimeMap {

    private Map<String, TreeMap<Integer, String>> timeMap;

    public TimeMap() {
        timeMap = new HashMap<>();
    }
    
    public void set(String key, String value, int timestamp) {
        if(!timeMap.containsKey(key)){
            timeMap.put(key, new TreeMap<>());
        }
        timeMap.get(key).put(timestamp, value);
    }
    
    public String get(String key, int timestamp) {
        if(!timeMap.containsKey(key)){
            return "";
        }
        //使用floorKey(timestamp)方法來找到小於或等於指定時間戳的最大時間戳（prevTimestamp）
        Integer prevTimestamp = timeMap.get(key).floorKey(timestamp);

        if(prevTimestamp == null){
            return "";
        }
        return timeMap.get(key).get(prevTimestamp);
    }
}

/**
 * Your TimeMap object will be instantiated and called as such:
 * TimeMap obj = new TimeMap();
 * obj.set(key,value,timestamp);
 * String param_2 = obj.get(key,timestamp);
 */

Time: O(N) Space: O(N)

<補充> TreeMap，並使用自定義比較器實現降序排列


import java.util.*;

public class TreeMapDemo {

    public static void main(String[] args) {
        // 創建一個 TreeMap，並使用自定義比較器實現降序排列
        TreeMap<Integer, String> descendingTreeMap = new TreeMap<>(Collections.reverseOrder());

        // 添加元素到 TreeMap
        descendingTreeMap.put(3, "Three");
        descendingTreeMap.put(1, "One");
        descendingTreeMap.put(2, "Two");

        // 遍歷 TreeMap，元素將按照降序排列
        for (Map.Entry<Integer, String> entry : descendingTreeMap.entrySet()) {
            System.out.println(entry.getKey() + ": " + entry.getValue());
        }
    }
}

Leetcode-981. Time Based Key-Value Store

<解題>

<補充> TreeMap，並使用自定義比較器實現降序排列