How to Fix Activity Selection Errors

In this tutorial, you'll learn about How to Fix Activity Selection Errors. We cover key concepts, practical examples, and best practices.

Fix activity selection errors when sort by end time vs start time confusion or no sorting before greedy.

Quick Fix

Wrong

def act(activities):
    activities.sort()  # sorts by start time, not end time!
    count=1; end=activities[0][1]
    for s,e in activities[1:]:
        if s>=end: count+=1; end=e
    return count

Sorting by start time fails. Example: [[1,10],[2,3],[4,5]] sorted by start gives 1 activity, optimal is 2.

Right

def act(activities):
    activities.sort(key=lambda x:x[1])  # sort by end time
    count=1; end=activities[0][1]
    for s,e in activities[1:]:
        if s>=end: count+=1; end=e
    return count

[[1,2],[3,4],[0,6],[5,7],[8,9],[5,9]] -> 4 activities. O(n log n).

Prevention

Sort by end time. Pick first activity. For each subsequent, if start >= last end, pick it.

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