[LeetCode] 981. Time Based Key-Value Store

[frdmu]

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 "ba2r" along with timestamp = 4.
timeMap.get("foo", 4);         // return "bar2"
timeMap.get("foo", 5);         // return "bar2"

Constraints:

• 1 <= key.length, value.length <= 100
• key and value consist of lowercase English letters and digits.
• 1 <= timestamp <= 107
• All the timestamps timestamp of set are strictly increasing.
• At most 2 * 105 calls will be made to set and get.

思路很明确，主要就是确定数据结构，我用的是map<string, map<int, string>>，代码如下：

class TimeMap {
public:
    /** Initialize your data structure here. */
    map<string, map<int, string>> mp; 
    TimeMap() {

    }
    
    void set(string key, string value, int timestamp) {
        mp[key][timestamp] = value;
    }
    
    string get(string key, int timestamp) {
        if (mp.count(key) == 0)
            return "";
        if (mp[key].count(timestamp) != 0)
            return mp[key][timestamp];
        int tmp = timestamp-1; 
        while (tmp != 0) {
            if (mp[key].find(tmp) != mp[key].end()) {
                break;
            }
            tmp--;
        } 
        return mp[key][tmp];
    }
};

/**
 * 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);
 */

时空间效率不高，下次再遇到的话，再改进。