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# 15.5 Summary

- Greedy algorithms are often used to solve optimization problems, where the principle is to make locally optimal decisions at each decision stage in order to achieve a globally optimal solution.
- Greedy algorithms iteratively make one greedy choice after another, transforming the problem into a smaller sub-problem with each round, until the problem is resolved.
- Greedy algorithms are not only simple to implement but also have high problem-solving efficiency. Compared to dynamic programming, greedy algorithms generally have a lower time complexity.
- In the problem of coin change, greedy algorithms can guarantee the optimal solution for certain combinations of coins; for others, however, the greedy algorithm might find a very poor solution.
- Problems suitable for greedy algorithm solutions possess two main properties: greedy-choice property and optimal substructure. The greedy-choice property represents the effectiveness of the greedy strategy.
- For some complex problems, proving the greedy-choice property is not straightforward. Contrarily, proving the invalidity is often easier, such as with the coin change problem.
- Solving greedy problems mainly consists of three steps: problem analysis, determining the greedy strategy, and proving correctness. Among these, determining the greedy strategy is the key step, while proving correctness often poses the challenge.
- The fractional knapsack problem builds on the 0-1 knapsack problem by allowing the selection of a part of the items, hence it can be solved using a greedy algorithm. The correctness of the greedy strategy can be proved by contradiction.
- The maximum capacity problem can be solved using the exhaustive method, with a time complexity of \(O(n^2)\). By designing a greedy strategy, each round moves inwardly shortening the board, optimizing the time complexity to \(O(n)\).
- In the problem of maximum product after cutting, we deduce two greedy strategies: integers \(\geq 4\) should continue to be cut, with the optimal cutting factor being \(3\). The code includes power operations, and the time complexity depends on the method of implementing power operations, generally being \(O(1)\) or \(O(\log n)\).

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