# Leetcode 1220 - Count Vowels Permutation

Note:

• Since from the end char of prev permutation, we can get next permutation. It looks like a DP problem!
• Let dp[i][j] be the number of permutations of a string with length i and ending with vowel[j].
• From the rules, we can deduct that
• a follows e, i, u.
• e follows a, i.
• i follows e, o.
• o follows i.
• u follows o, i.
• For example, if the vowel is a, then it must appear after e, i, u, so we can just add the number of permutations that end with e, i, u. Then what we get would be the number of permutations ending with a.

Question:

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.

Example:

Code: