981. Time Based Key-Value Store

Problem

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"

  Constraints:

• 1 <= key.length, value.length <= 100

• key and value consist of lowercase English letters and digits.

• 1 <= timestamp <= 10^7

• All the timestamps timestamp of set are strictly increasing.

• At most 2 * 10^5 calls will be made to set and get.

Solution (Java)

class TimeMap {
    private Map<String, List<TimeStampData>> map;

    private static class TimeStampData {
        int timestamp;
        String value;

        TimeStampData(int timestamp, String value) {
            this.timestamp = timestamp;
            this.value = value;
        }
    }

    public TimeMap() {
        this.map = new HashMap<>();
    }

    public void set(String key, String value, int timestamp) {
        List<TimeStampData> timeStampDataList;
        if (!this.map.containsKey(key)) {
            timeStampDataList = new ArrayList<>();
            timeStampDataList.add(new TimeStampData(timestamp, value));
            map.put(key, timeStampDataList);
        } else {
            this.map.get(key).add(new TimeStampData(timestamp, value));
        }
    }

    public String get(String key, int timestamp) {
        if (!this.map.containsKey(key)) {
            return "";
        }
        List<TimeStampData> list = this.map.get(key);
        int start = 0;
        int end = list.size() - 1;
        while (start <= end) {
            int mid = start + (end - start) / 2;
            if (list.get(mid).timestamp == timestamp) {
                return list.get(mid).value;
            }
            if (timestamp > list.get(mid).timestamp) {
                start = mid + 1;
            } else {
                end = mid - 1;
            }
        }
        return end < 0 ? "" : list.get(end).value;
    }
}

/**
 * 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);
 */

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).