Grokking Amazon Coding Interview
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Solution: Count Vowels Permutation

Problem Statement

Given a positive integer n, return the number of strings of length n can be formed by following rules:

  • Each character of the string should be only a lowercase vowel ('a', 'e', 'i', 'o', 'u').
  • The vowel 'a' is only allowed to be followed by the vowel 'e'.
  • The vowel 'e' can be followed by either 'a' or 'i'.
  • The vowel 'i' cannot be followed by another 'i'.
  • The vowel 'o' can be followed by either 'i' or 'u'.
  • The vowel 'u' must be followed by 'a'.

Return the answer modulo 10^9 + 7, as it could be very large.

Examples

Example 1:

  • Input: `n = 1

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