sliding window

A dashboard needs the average tokens/sec over the last N seconds, updated on every sample, forever. The warm-up that screens whether you evict by time and keep a running sum instead of re-summing the window each call.

A sliding window with a last-seen index map, O(n) in one pass. The whole interview turns on one subtlety: when you hit a repeat, the window's left edge must jump forward and never slide backward, which is exactly the bug most candidates ship.