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.

• 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 "".

timeMap.set('foo', 'bar', 1);
timeMap.get('foo', 1); //returns 'bar'
timeMap.get('foo', 3); //returns 'bar'
timeMap.set('foo', 'baz', 3);
timeMap.get('foo', 1); // returns 'bar'
timeMap.get('foo', 2); // returns 'bar'
timeMap.get('foo', 3); // returns 'baz'

• key and value are strings
• The timestamp given in set are strictly increasing.