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# 15.5 Summary

### 1. Key Review

- Greedy algorithms are typically used to solve optimization problems. The principle is to make locally optimal decisions at each decision stage in hopes of obtaining a globally optimal solution.
- Greedy algorithms iteratively make one greedy choice after another, transforming the problem into a smaller subproblem in each round, until the problem is solved.
- Greedy algorithms are not only simple to implement, but also have high problem-solving efficiency. Compared to dynamic programming, greedy algorithms typically have lower time complexity.
- In the coin change problem, for certain coin combinations, greedy algorithms can guarantee finding the optimal solution; for other coin combinations, however, greedy algorithms may find very poor solutions.
- Problems suitable for solving with greedy algorithms have two major 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 simple. Relatively speaking, disproving it is easier, such as in the coin change problem.
- Solving greedy problems mainly consists of three steps: problem analysis, determining the greedy strategy, and correctness proof. Among these, determining the greedy strategy is the core step, and correctness proof is often the main difficulty.
- The fractional knapsack problem, based on the 0-1 knapsack problem, allows selecting fractions of items, and therefore can be solved using greedy algorithms. The correctness of the greedy strategy can be proven using proof by contradiction.
- The max capacity problem can be solved using exhaustive enumeration with time complexity \(O(n^2)\). By designing a greedy strategy to move the shorter side inward in each round, the time complexity can be optimized to \(O(n)\).
- In the max product cutting problem, we successively derive two greedy strategies: integers \(\geq 4\) should all continue to be split, and the optimal splitting factor is \(3\). The code includes exponentiation operations, and the time complexity depends on the implementation method of exponentiation, typically being \(O(1)\) or \(O(\log n)\).

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