1220. Count Vowels Permutation

README.md

@@ -320,6 +320,7 @@
 | 1203 | Sort Items by Groups Respecting Dependencies | [Ruby](./algorithms/ruby/1203-sort-items-by-groups-respecting-dependencies.rb) | Hard |
 | 1207 | Unique Number of Occurrences | [Ruby](./algorithms/ruby/1207-unique-number-of-occurrences.rb) | Easy |
 | 1218 | Longest Arithmetic Subsequence of Given Difference | [Ruby](./algorithms/ruby/1218-longest-arithmetic-subsequence-of-given-difference.rb) | Medium |
+| 1220 | Count Vowels Permutation | [Ruby](./algorithms/ruby/1220-count-vowels-permutation.rb) | Hard |
 | 1232 | Check If It Is a Straight Line | [Ruby](./algorithms/ruby/1232-check-if-it-is-a-straight-line.rb) | Easy |
 | 1254 | Number of Closed Islands | [Ruby](./algorithms/ruby/1254-number-of-closed-islands.rb) | Medium |
 | 1268 | Search Suggestions System | [Ruby](./algorithms/ruby/1268-search-suggestions-system.rb) | Medium |

algorithms/ruby/1220-count-vowels-permutation.rb

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+# frozen_string_literal: true
+
+# 1220. Count Vowels Permutation
+# Hard
+# https://leetcode.com/problems/count-vowels-permutation
+
+=begin
+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 1:
+Input: n = 1
+Output: 5
+Explanation: All possible strings are: "a", "e", "i" , "o" and "u".
+
+Example 2:
+Input: n = 2
+Output: 10
+Explanation: All possible strings are: "ae", "ea", "ei", "ia", "ie", "io", "iu", "oi", "ou" and "ua".
+
+Example 3:
+Input: n = 5
+Output: 68
+
+Constraints:
+1 <= n <= 2 * 10^4
+=end
+
+# @param {Integer} n
+# @return {Integer}
+def count_vowel_permutation(n)
+ a = e = i = o = u = 1
+ (2..n).each do
+ a, e, i, o, u = e + i + u, a + i, e + o, i, i + o
+ end
+ (a + e + i + o + u) % 1000000007
+end
+
+# **************** #
+# TEST #
+# **************** #
+
+require "test/unit"
+class Test_count_vowel_permutation < Test::Unit::TestCase
+ def test_
+ assert_equal 5, count_vowel_permutation(1)
+ assert_equal 10, count_vowel_permutation(2)
+ assert_equal 68, count_vowel_permutation(5)
+ end
+end