#1220 - Count Vowels Permutation

Problem 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.

Solution

/**
 * @param {number} n
 * @return {number}
 */
var countVowelPermutation = function(n) {
  const MOD = 1000000007n;
  let aCount = 1n;
  let eCount = 1n;
  let iCount = 1n;
  let oCount = 1n;
  let uCount = 1n;

  for (let length = 2; length <= n; length++) {
    const prevACount = aCount;
    const prevECount = eCount;
    const prevICount = iCount;
    const prevOCount = oCount;
    const prevUCount = uCount;

    aCount = (prevECount + prevICount + prevUCount) % MOD;
    eCount = (prevACount + prevICount) % MOD;
    iCount = (prevECount + prevOCount) % MOD;
    oCount = (prevICount) % MOD;
    uCount = (prevICount + prevOCount) % MOD;
  }

  const total = aCount + eCount + iCount + oCount + uCount;

  return Number(total % MOD);
};