LeetCode: Count Vowels Permutation Solution

Approach

recursion(i, char) number of valid strings of n - i length, starting with char

Implementation

1/**
2 * @param {number} n
3 * @return {number}
4 */
5var countVowelPermutation = function (n) {
6  const MOD = 1e9 + 7
7
8  const possibleNext = {
9    a: "e",
10    e: "ai",
11    i: "aeou",
12    o: "iu",
13    u: "a",
14  }
15
16  const memo = Array.from({ length: n }, _ => ({}))
17
18  const recursion = (i, char, possibleChars = "aeiou") => {
19    if (i === n) return 1
20
21    if (memo[i][char]) return memo[i][char]
22
23    let res = 0
24
25    for (const possibleChar of possibleChars) {
26      res =
27        (res + recursion(i + 1, possibleChar, possibleNext[possibleChar])) % MOD
28    }
29
30    return (memo[i][char] = res)
31  }
32
33  return recursion(0)
34}