Employee Free Time

1. Flatten All Intervals

• Iterate through every employee's schedule and collect all intervals into a single flat list
• We don't care which employee an interval belongs to — we only care about overall coverage

2. Sort by Start Time

• Sort all the flattened intervals by their start value in ascending order
• This sets up the merge step by ensuring overlapping intervals are adjacent

3. Merge Overlapping Intervals

• Initialize a merged array with the first sorted interval
• For each subsequent interval, compare its start with the last merged interval's end
• If overlapping (curr[0] <= last[1]): extend the end to Math.max(last[1], curr[1])
• If not overlapping (curr[0] > last[1]): push it as a new merged interval

4. Find Gaps Between Merged Intervals

• After merging, any gap between consecutive merged intervals is a period when no employee is working
• For each pair of consecutive merged intervals, the gap is [merged[i-1][1], merged[i][0]]
• These gaps are the common free time

5. Return Result

• The gaps are already in sorted order because the merged intervals are sorted
• Return the list of free time intervals

Key Insight

This problem reduces to two well-known sub-problems: merge intervals followed by gap finding. Once you flatten all employee schedules into one list and merge overlapping intervals, the free time is simply the space between consecutive merged intervals. The per-employee structure is a red herring — it doesn't affect the algorithm at all.