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Answers for “动态规划问题的两个定义特征是什么?”

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A problem is a dynamic programming problem if it satisfies two conditions: optimal substructure and overlapping subproblems. Optimal substructure means the optimal solution to the overall problem can be constructed from the optimal solutions of its subproblems.

Conditions: The problem can be solved using an algorithmic approach.; The problem can be divided into subproblems.

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The Fibonacci recursion tree demonstrates repeated computation by showing that the same subproblems are solved multiple times. For example, when computing the fifth Fibonacci number, the tree contains duplicated subcalls such as fib(2) and fib(3).

Conditions: The problem is being solved recursively.; The recursive decomposition revisits the same subproblem multiple times.

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Overlapping subproblems occur when the same smaller problem is solved multiple times during a recursive process. The Fibonacci tree makes this concrete by showing repeated calls such as fib(2) and fib(3), which cause redundant computation.

Conditions: The problem can be solved recursively.; The same subproblems appear multiple times in the recursion tree.

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The San Francisco to San Diego route can be split at Los Angeles because the problem exhibits optimal substructure. Once the shortest SF→LA and LA→SD segments are known, they combine to give the shortest SF→SD route.

Conditions: The example assumes the overall route passes through Los Angeles.; The narrator states that finding the shortest subroutes guarantees the overall shortest route in this example.

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Richard Bellman used the term 'programming' to signify planning and decision making, not writing computer code. The term 'dynamic' refers to the ever-changing nature of the problems being solved.

Conditions: The context is the historical origin of the term 'dynamic programming'.

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To determine if a problem can be solved with dynamic programming, first try to solve the problem recursively. Then check whether divide-and-conquer style decomposition works, and whether breaking the problem down to the simplest subproblem can eventually build up to the optimal solution.

Conditions: You are trying to decide whether a problem is suitable for dynamic programming.

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The factorial example illustrates the core mechanism of dynamic programming by showing how a complex problem is recursively broken down into smaller subproblems and then built back up. To compute 4!4!, it depends on 3!3!, which depends on 2!2!, which depends on 1!1!.

Conditions: The problem can be broken down into smaller subproblems.; The solution to the original problem can be constructed from the solutions to the subproblems.

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Optimal substructure means that the optimal solution to the overall problem can be constructed by efficiently combining the optimal solutions of its subproblems. In the route example, once the shortest SF→LA and LA→SD segments are known, they combine to give the shortest SF→SD route.

Conditions: The problem can be divided into subproblems.; The optimal solution of the main problem depends on the optimal solutions of its subproblems.