Recent Calls - Algorithm

Use a queue to store timestamps of requests:

$1.$ When ping(t) is called, enqueue $t$.

$2.$ Dequeue all timestamps less than $t - 3000$ (they're too old).

$3.$ Return the queue size. The queue always contains exactly the requests in the valid time window. Older requests get removed as newer ones arrive. Amortized time: $O(1)$ per call. Space: $O(n)$.