Recent Calls - Implementation

Here's the solution:

class RecentCounter
    queue := empty queue

    function ping(t)
        enqueue t

        // Remove timestamps outside the window
        while front of queue < t - 3000
            dequeue

        return size of queue

Each timestamp is enqueued once and dequeued at most once. If there are $n$ calls to ping, total time is $O(n)$. Amortized $O(1)$ per call.