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981. Time Based Key-Value Store

Problem: Time Based Key Value Store

Understanding: This is a fascinating problem requiring us to design a new kind of map/database that could retrieve a key: String by timestamp: int.

The tricky part of the problem is that it requires us to retrieve data of a key such that the return timestamp is less than or equal to the requested timestamp.

For example, if we have a key: foo with the following timestamp: 2: "a", 4: "b", 6: "c". This means that timestamp 2 will be matched with value a, 4 be matched with value b and 6 be matched with value c.

So, if the request is get(foo, 2) we can simply return a. Similarly get(foo, 3) should also return a since 2 is already the closest timestamp to 3.
And if the request is get(foo, 5), that should return b. However, if the request is get(foo, 1), we should return "" since there is no timestamp available that is smaller than or equal to 1.

Matching: We want to match this problem to a DataStructure or Algorithm that we already know

  1. Data Structure: since we want a fast retrieval runtime, HashMap (or dictionary) is the best choice. And in order to retrieve value as map.get(key).get(timestamp), the type of the map should be Map<String, Map<Integer, String>>
  2. Algorithm: there is an interesting find when we look at the input constraints. The timestamp value passed into the set() function will be strictly increased. That means if we keep track of the a list of timestamp, that list will have a strictly ascending order. This simply remind us of Binary Search since we have a sorted array (list).


We need to have a map of type Map<String, Map<Integer, String>> as discussed above.

But we also need another map of type Map<String, List<Integer>>. This map will store a list of timestamps connected with a specific key in ascending order. This will be used as source data for binary search.

class TimeMap {
    Map<String, Map<Integer, String>> keyValueMap;
    Map<String, List<Integer>> keyTimeMap;
    public TimeMap() {
        this.keyValueMap = new HashMap<>();
        this.keyTimeMap = new HashMap<>();

    public void set(String key, String value, int timestamp) {
        this.keyValueMap.putIfAbsent(key, new HashMap<>());
        this.keyValueMap.get(key).put(timestamp, value);

        // create a list of time always in ascending order
        this.keyTimeMap.putIfAbsent(key, new ArrayList<>());

    private Integer findIndex(String key, int timestamp) {
        // return -1 if we find nothing
        // return i if time at i <= timestamp and time at i + 1 > time 
        // if timestamp < first element in the list => return -1
        // if time stamp >= last element in the list, return the last index 

        // 1. check if the list exist
        if (!this.keyTimeMap.containsKey(key)) {
            return -1;

        // 2. get the list and check the edge case
        List<Integer> timeList = this.keyTimeMap.get(key);
        int left = 0;
        int right = timeList.size() - 1;

        if (timestamp < timeList.get(left)) {
            return -1;
        } else if (timestamp >= timeList.get(right)) {
            return timeList.get(right);
        } else {
            while (left <= right) {
                int middle = (left + right) / 2;

                if (middle <= timeList.size() - 1 && timeList.get(middle) <= timestamp && timeList.get(middle + 1) > timestamp) {
                    return timeList.get(middle);
                } else if (timestamp > timeList.get(middle)) { // lef side is useless
                    left = middle + 1;
                } else if (timestamp < timeList.get(middle)) { // right side is useless
                    right = middle - 1;
            return -1;


    public String get(String key, int timestamp) {
        Utilize binary search to find a place in the middle of the list in which index i <= time and index i + 1 > time. return i;
        // find the index of the current timestamp 
        int timeValue = this.findIndex(key, timestamp);

        if (timeValue < 0) {
            return "";
        } else {
            return this.keyValueMap.get(key).get(timeValue);

 * 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);
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The constructor function TimeMap() should be easy to understand since we just initialize the maps needed for our implementation.

Similarly, set() method is just putting elements into the map like we discussed above.

The part that might be a little bit confusing is the findIndex(String key, int timestamp) function which is trying the find the time value that is closest to the requested timestamp.

  • if there is no existing key stored in our database, return -1. This denoted that there is no valid time value. Else, we have a list variable storing all timestamps value already get put into the database connecting with the key key.
  • if the value of timestamp is strictly smaller than the first value in list, we can also return -1 since there is no other suitable value.
  • if the value of timestamp is larger than or equal to the last element in list, we return that last element since that is already the largest value we could possibly get.
  • otherwise, we can start our binary search. The main go is to find an index i at which list.get(i) <= timestamp and list.get(i + 1) > timestamp. This will shows that list.get(i) is the maximum time value we can get.

For example, assume that we have a request findIndex("foo", 5) and we have an associated list of timestamp connecting with key "foo" is: [1, 2, 3, 4, 6, 7].

  • start binary search with left = 0 and right = 5.
  • start 1st while: middle = 2. list.get(2) = 3 This value does not satisfy anything and requires us to ignore anything on the left of middle (anything smaller than index 2). So: left = 3 and start new while loop
  • start 2st while with left = 3, right = 5: middle = (3 + 5) / 2 = 4. list.get(4) = 6 this also does not satisfy our checks and 6 > 5 so we ignore anything larger than 6. So: right = middle - 1 = 3 and continue the while
  • start 3rd while with left = right = 3: middle = 3 and list.get(3) = 4. This satisfies our check since list.get(3) = 4 <= 5 and list.get(3 + 1) = 6 > 5. So we return 4. Finally, we just have to return map.get("foo").get(4) as the final result

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