1641. Count Sorted Vowel Strings

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

1641. Count Sorted Vowel Strings
struct Solution;

impl Solution {
fn count_vowel_strings(n: i32) -> i32 {
let n = n as usize;
let mut res = 0;
for i in 0..5 {
res += Self::dp(i, n);
}
res
}

fn dp(i: i32, n: usize) -> i32 {
if n == 1 {
1
} else {
let mut res = 0;
for j in 0..=i {
res += Self::dp(j, n - 1);
}
res
}
}
}

#[test]
fn test() {
let n = 1;
let res = 5;
assert_eq!(Solution::count_vowel_strings(n), res);
let n = 2;
let res = 15;
assert_eq!(Solution::count_vowel_strings(n), res);
let n = 33;
let res = 66045;
assert_eq!(Solution::count_vowel_strings(n), res);
}