1220. Count Vowels Permutation

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

# @lc code=start
using LeetCode

function count_vowel_permutation(n::Int)::Int
    table = Dict{Char,Int}(i => 1 for i in "aeiou")
    quotient = 10^9 + 7
    for _ in 1:(n - 1)
        a = table['e'] + table['i'] + table['u']
        e = table['a'] + table['i']
        i = table['e'] + table['o']
        o = table['i']
        u = table['i'] + table['o']
        table['a'], table['e'], table['i'], table['o'], table['u'] = map(
            x -> mod(x, quotient), [a, e, i, o, u]
        )
    end
    return mod(sum(values(table)), quotient)
end

# @lc code=end

count_vowel_permutation (generic function with 1 method)

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