# 981. Time Based Key-Value Store | LeetCode Solution

## 981. Time Based Key-Value Store

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 <= 107`
• All the timestamps `timestamp` of `set` are strictly increasing.
• At most `2 * 105` calls will be made to `set` and `get`.
What to learn:

Imagine we have the following timestamps: `[5, 10, 15]`.

If we query, say, `20``upper_bound` returns `end()`, so the result is the label for `15` (`prev(end())`).
Now, if we query `3``upper_bound` returns `begin()`, and that means we do not have a label (the result is an empty string).

Another interesting case is when we query the exact timestamp, say, `10`. upper_bound will return `15` and `prev(15)` will gives us the correct label. We could have used `lower_bound`, but we will have to add an equality check.