Detect Anomalies with Sliding Window | Dataford Interview Questions

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Problem

Pearson MyLab records student interaction events in chronological order. Detect students whose activity exceeds an allowed interaction count within any fixed-size sliding time window.

An interaction log is represented as a list of two-item arrays: [timestamp, student_id]. Timestamps are integer seconds and the logs are sorted by nondecreasing timestamp. For each student, return the student ID if any window of window_size seconds contains more than max_events interactions.

Treat a window ending at timestamp t as the inclusive interval [t - window_size + 1, t]. Return the anomalous student IDs in lexicographic order. A student should appear at most once.

Formal Specification

Implement detect_anomalies(logs, window_size, max_events).

Input: logs, a list of [int, str] pairs; window_size and max_events, positive integers.
Output: a sorted list of student ID strings whose interaction count exceeds max_events in at least one window.

Constraints

0 <= len(logs) <= 10^5
Each log entry has the form [timestamp, student_id]
0 <= timestamp <= 10^9
1 <= window_size <= 10^9
1 <= max_events <= 10^5
Each student_id is a nonempty string
logs is sorted by nondecreasing timestamp

Function Signature

def detect_anomalies(logs, window_size, max_events):