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 

Solution:

	class Solution(object):
	def countVowelStrings(self, n):
	"""
	:type n: int
	:rtype: int
	"""
	dp ={}
	def cv(i,vowels):
	if i in dp and vowels in dp[i]:
	return dp[i][vowels]
	if i==1:
	return vowels
	if vowels ==1:
	return 1
	if i not in dp:
	dp[i]=dict()
	dp[i][vowels]=cv(i-1,vowels)+cv(i,vowels-1)
	return dp[i][vowels]
	return cv(n,5)

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