public class Solution
extends Object
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`