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g1201_1300.s1220_count_vowels_permutation.readme.md Maven / Gradle / Ivy

1220\. Count Vowels Permutation

Hard

Given an integer `n`, your task is to count how many strings of length `n` can be formed under the following rules:

*   Each character is a lower case vowel (`'a'`, `'e'`, `'i'`, `'o'`, `'u'`)
*   Each vowel `'a'` may only be followed by an `'e'`.
*   Each vowel `'e'` may only be followed by an `'a'` or an `'i'`.
*   Each vowel `'i'` **may not** be followed by another `'i'`.
*   Each vowel `'o'` may only be followed by an `'i'` or a `'u'`.
*   Each vowel `'u'` may only be followed by an `'a'.`

Since the answer may be too large, return it modulo `10^9 + 7.`

**Example 1:**

**Input:** n = 1

**Output:** 5

**Explanation:** All possible strings are: "a", "e", "i" , "o" and "u".

**Example 2:**

**Input:** n = 2

**Output:** 10

**Explanation:** All possible strings are: "ae", "ea", "ei", "ia", "ie", "io", "iu", "oi", "ou" and "ua".

**Example 3:**

**Input:** n = 5

**Output:** 68

**Constraints:**

*   `1 <= n <= 2 * 10^4`