[LeetCode] 981. Time Based Key-Value Store 基於時間的鍵值存儲

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.

這道題讓我們實現一種基於時間的鍵值對兒數據結構，有兩種操作 set 和 get，其中 set 就是存入鍵值對兒，同時需要保存時間戳，get 就是查找值，但此時不僅提供了 key 值，還提供了查詢的時間戳，返回值的時間戳不能大於查詢的時間戳，假如有多個相同值，返回時間戳最大的那個，若查詢不到就返回空。實際上這道題考察的就是較為復雜一些的數據結構，因為要同時保存三個量，而且還要提供快速查詢功能，可以使用 Map of Maps 的數據結構，外層可以使用一個 HashMap，因為對於 key 值沒有順序要求，而內層要使用一個 TreeMap，因為時間戳的順序很重要。在 set 函數中直接將數據插入數據結構中，在 get 中，用一個 upper_bound 來進行快速查找第一個大於目標值的位置，往后退一位，就是不大於目標值的位置。但是在退之前要判斷得到的位置是否是起始位置，是的話就沒法再往前退一位了，直接返回空串，不是的話可以退一位並返回即可，參見代碼如下：

class TimeMap {
public:
    TimeMap() {}
    
    void set(string key, string value, int timestamp) {
        dataMap[key].insert({timestamp, value});
    }
    
    string get(string key, int timestamp) {
        auto it = dataMap[key].upper_bound(timestamp);
        return it == dataMap[key].begin() ? "" : prev(it)->second;
    }

private:
    unordered_map<string, map<int, string>> dataMap;
};

Github 同步地址:

https://github.com/grandyang/leetcode/issues/981

參考資料：

https://leetcode.com/problems/time-based-key-value-store/

https://leetcode.com/problems/time-based-key-value-store/discuss/226663/TreeMap-Solution-Java

https://leetcode.com/problems/time-based-key-value-store/discuss/226664/C%2B%2B-3-lines-hash-map-%2B-map

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