[HackerEarth] Please find the number of N-length strings that are made of only letters 'a, 'b', 'c' and 'd' and do not contain any of the restricted strings. - 题解

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题号 P13984
标题 [HackerEarth] Please find the number of N-length strings that are made of only letters 'a, 'b', 'c' and 'd' and do not contain any of the restricted strings.
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