1641. Count Sorted Vowel Strings

#### QUESTION:

Given an integer n, return the number of strings of length n that consist only of vowels (a, e, i, o, u) and are lexicographically sorted.

A string s is lexicographically sorted if for all valid i, s[i] is the same as or comes before s[i+1] in the alphabet.

Example 1:

``````Input: n = 1
Output: 5
Explanation: The 5 sorted strings that consist of vowels only are ["a","e","i","o","u"].
``````

Example 2:

``````Input: n = 2
Output: 15
Explanation: The 15 sorted strings that consist of vowels only are
["aa","ae","ai","ao","au","ee","ei","eo","eu","ii","io","iu","oo","ou","uu"].
Note that "ea" is not a valid string since 'e' comes after 'a' in the alphabet.
``````

Example 3:

``````Input: n = 33
Output: 66045
``````

Constraints:

``````1 <= n <= 50
``````

#### SOLUTION:

``````class Solution {
func countVowelStrings(_ n: Int) -> Int {
var a = 1, e = 1 , i = 1 , o = 1 , u = 1
for index in 1..<n {
a = a + e + i + o + u
e = e + i + o + u
i = i + o + u
o = o + u
}
return a + e + i + o + u
}
}
``````