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`.