How to Solve Cache With Time Limit Problem

Write a class that allows getting and setting key-value pairs, however a time until expiration is associated with each key. The class has three public methods: set(key, value, duration): accepts an integer key, an integer value, and a duration in milliseconds. Once the duration has elapsed, the key should be inaccessible. The method should return true if the same un-expired key already exists and false otherwise. Both the value and duration should be overwritten if the key already exists. get(key): if an un-expired key exists, it should return the associated value. Otherwise it should return -1. count(): returns the count of un-expired keys.