Time Based Key-Value Store

Create a timebased key-value store class TimeMap, that supports two operations.

1. set(string key, string value, int timestamp)

• Stores the key and value, along with the given timestamp.

2. get(string key, int timestamp)

• Returns a value such that set(key, value, timestamp_prev) was called previously, with timestamp_prev <= timestamp.
• If there are multiple such values, it returns the one with the largest timestamp_prev.
• If there are no values, it returns the empty string ("").

 

Example 1:

Input: inputs = ["TimeMap","set","get","get","set","get","get"], inputs = [[],["foo","bar",1],["foo",1],["foo",3],["foo","bar2",4],["foo",4],["foo",5]]
Output: [null,null,"bar","bar",null,"bar2","bar2"]
Explanation:   
TimeMap kv;   
kv.set("foo", "bar", 1); // store the key "foo" and value "bar" along with timestamp = 1   
kv.get("foo", 1);  // output "bar"   
kv.get("foo", 3); // output "bar" since there is no value corresponding to foo at timestamp 3 and timestamp 2, then the only value is at timestamp 1 ie "bar"   
kv.set("foo", "bar2", 4);   
kv.get("foo", 4); // output "bar2"   
kv.get("foo", 5); //output "bar2"   

Example 2:

Input: inputs = ["TimeMap","set","set","get","get","get","get","get"], inputs = [[],["love","high",10],["love","low",20],["love",5],["love",10],["love",15],["love",20],["love",25]]
Output: [null,null,null,"","high","high","low","low"]

 

Note:

1. All key/value strings are lowercase.
2. All key/value strings have length in the range [1, 100]
3. The timestamps for all TimeMap.set operations are strictly increasing.
4. 1 <= timestamp <= 10^7
5. TimeMap.set and TimeMap.get functions will be called a total of 120000 times (combined) per test case.

题目理解：

要求给出一个字典类，这个字典里面，既需要关键字和值，还需要时间戳信息，查询的时候也是即需要关键字，也需要时间戳，查询范围关键字匹配且时间戳小于或等于给定时间的最大时间戳值

解题思路：

使用两层的Map，外层是简单的HashMap，用key对应一个TreeMap，TreeMap里面用timestamp对应value，查询时先使用key查询外层的HashMap，再在内层的TreeMap里面查找合适的时间戳。使用TreeMap的原因是，它可以很方便的查到小于给定时间的最大时间戳。

class TimeMap {

    /** Initialize your data structure here. */
    Map> map;
    public TimeMap() {
        map = new HashMap>();
    }
    
    public void set(String key, String value, int timestamp) {
        if(!map.containsKey(key))
            map.put(key, new TreeMap());
        TreeMap timeMap = map.get(key);
        timeMap.put(timestamp, value);
    }
    
    public String get(String key, int timestamp) {
        if(!map.containsKey(key))
            return "";
        TreeMap timeMap = map.get(key);
        if(timeMap.floorKey(timestamp) != null)
            return timeMap.get(timeMap.floorKey(timestamp));
        return "";
    }
}

/**
 * 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);
 */