The naive implementation of this recurrence relation as a recursive method would have led to an inefficient solution involving multiple computations of subproblems.
用递归方法对这些递归关系的直接实现会造成解决方案效率低下，因为其中包含了对子问题的多次计算。
The solution to each of them could be expressed as a recurrence relation.
每个问题的解都能用递归关系表示。
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