Issue Non-repeating OTPs in a Time Window

Given a nondecreasing integer array requestTimes and a positive integer window, issue one four-digit integer code for each request. Codes range from 1000 through 9999.
At each request, choose the smallest available code. A code issued at time t is unavailable during the half-open interval [t, t + window) and becomes available again exactly at t + window. Every reuse starts a new interval.
Process requests with equal timestamps in their input order. Return the issued codes in request order. The request-count bound guarantees that a code is always available. An empty request list returns an empty array.

Function
issueOtps(requestTimes: int[], window: int) → int[]

Examples
Example 1
requestTimes = [0,1,2,5,6]
window = 5
return = [1000,1001,1002,1000,1001]
At time 5, code 1000 expires and becomes the smallest available code. Code 1001 similarly expires at time 6.
Example 2
requestTimes = [4,4,4,7]
window = 3
return = [1000,1001,1002,1000]
Equal-time requests receive distinct codes in input order. All three codes expire at time 7, when the smallest can be reused.

Constraints
0 ≤ requestTimes.length ≤ 1000.
0 ≤ requestTimes[i] ≤ 10^9.
requestTimes is nondecreasing.
1 ≤ window ≤ 10^9.

The trick is two structures. Keep a min-heap of available codes, seeded with 1000 through 9999. Keep a FIFO queue of (expiryTime, code) pairs. Since requestTimes is nondecreasing, expiries are also nondecreasing, so a plain queue works and you don't need a second heap. For each request at time t, pop every queue entry with expiry <= t and push its code back into the heap. Then pop the smallest code, record it, and enqueue (t + window, code). The pitfall is using < instead of <= at the boundary. Example 1 shows code 1000 reused at time 5 with window 5. Another trap is scanning all 9000 codes per request, which is wasteful. Equal timestamps need no special handling, because the heap hands out distinct codes in order. Total cost is O(n log 9000). StealthCoder is your hedge in the live OA if the expiry order slips your mind mid-run.

Memorize the pattern. If you can't, run StealthCoder. The proctor sees the IDE. They don't see what's behind it.