1220. Count Vowels Permutation

https://leetcode.com/problems/count-vowels-permutation

Description

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

ac