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hardDSA

What is a greedy algorithm?

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01

Understand the problem

Explain greedy and when it works.

greedy
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A greedy algorithm makes the locally optimal choice at each step, hoping for a global optimum. It works only when the problem has the greedy-choice property and optimal substructure (e.g. interval scheduling, Huffman coding, Dijkstra). Otherwise it can fail, and DP is needed.

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04

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Activity selection (interval scheduling)
Run Playground
def max_activities(intervals):
    # intervals = list of (start, end)
    intervals.sort(key=lambda x: x[1])   # by finish time
    count, last_end = 0, float('-inf')
    for start, end in intervals:
        if start >= last_end:
            count += 1
            last_end = end
    return count


# --- demo ---
print(max_activities([(1, 3), (2, 5), (4, 7), (6, 9), (8, 10)]))   # 3
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