Design and implement an LRU (Least Recently Used) cache. The cache should support two operations: get(key) and put(key, value). Both operations must run in O(1) time on average. If the cache exceeds its capacity, the least recently used key should be evicted.
You are tasked with simulating the history of web page visits in a browser. Implement the BrowserHistory class to manage visit, back, and forward actions.
You are tasked with creating an authentication system that handles tokens. Each token has an expiration time, and a user can generate new tokens or renew existing ones before they expire. The system should track and count the number of unexpired tokens at any given time.
