High-Access Employees

High-Access Employees

What is this problem about?

The High-Access Employees coding problem tasks you with identifying employees who accessed a secure system multiple times within a short duration. You are given a list of access logs, each containing an employee's name and the time of access (in "HHMM" format). An employee is considered "high-access" if they have three or more access events within any 60-minute window.

Why is this asked in interviews?

Companies like Goldman Sachs and Atlassian ask this to test your ability to process time-series data and group information. It evaluates your mastery of Hash Table interview patterns for grouping and Sliding Window interview patterns for checking intervals. It’s a practical security-monitoring scenario that reflects real-world backend logic.

Algorithmic pattern used

The problem follows a Grouping, Sorting, and Sliding Window approach:

1. Group: Use a Hash Map to store a list of access times for each employee.
2. Convert: Convert the "HHMM" strings into total minutes from the start of the day ($hour imes 60 + minutes$) to make math easier.
3. Sort: Sort the list of access times for each employee chronologically.
4. Check: For each employee, iterate through their sorted times. Check if time[i] - time[i-2] < 60. If this condition is met at any point, the employee is high-access.

Example explanation

Employee "Alice" access times: ["1000", "1020", "1050", "1110"].

1. Minutes: [600, 620, 650, 670].
2. Sorted: [600, 620, 650, 670].
3. Check window of size 3:
   • Index 2: $650 - 600 = 50$. Since $50 < 60$, Alice is high-access. Even though Alice has 4 accesses, the first three already triggered the rule.

Common mistakes candidates make

• String Sorting: Sorting the "HHMM" strings directly without converting to integers (works for some cases but risky).
• Interval Logic: Using $\le 60$ instead of $< 60$ (the problem usually specifies "within" an hour, meaning the difference must be strictly less than 60).
• Inefficient Search: Trying to check all pairs of times instead of just checking the boundaries of a 3-element window in a sorted list.

Interview preparation tip

Always convert time strings to a single unit (like minutes or seconds) immediately. It simplifies the logic from complex time arithmetic to basic integer subtraction.