From e4a6a74e1837edc339d635b6ea555fce16d812ec Mon Sep 17 00:00:00 2001
From: Lam Haoyin <root@eatswap.org>
Date: Mon, 17 Jan 2022 00:43:53 +0800
Subject: [PATCH] add: 220117

---
 2201/220117-CN.cpp  | 59 +++++++++++++++++++++++++++++++++++++++++++++
 2201/CMakeLists.txt |  2 +-
 2 files changed, 60 insertions(+), 1 deletion(-)
 create mode 100644 2201/220117-CN.cpp

diff --git a/2201/220117-CN.cpp b/2201/220117-CN.cpp
new file mode 100644
index 0000000..1f1954c
--- /dev/null
+++ b/2201/220117-CN.cpp
@@ -0,0 +1,59 @@
+#include <iostream>
+
+/**
+ * 1220. Count Vowels Permutation
+ * 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.
+ */
+
+class Solution {
+private:
+	static const int mod = 1000000007;
+	// dp[currentLength][endsWith]
+	// int dp[20005][5]{{}, {1, 1, 1, 1, 1}};
+	/**
+	 * Rules of assembling characters:
+	 * 'a' can be derived from those that ends with 'eiu'.
+	 * 'e' <- 'ai'
+	 * 'i' <- 'eo'
+	 * 'o' <- 'i'
+	 * 'u' <- 'io'
+	 */
+public:
+	static int countVowelPermutation(int n) {
+		int dp[5]{1, 1, 1, 1, 1}, tmp[5];
+		for (int i = 2; i <= n; ++i) {
+			// 'a' <- 'eiu'
+			tmp[0] = ((dp[1] + dp[2]) % mod + dp[4]) % mod;
+
+			// 'u' <- 'io'
+			dp[4] = (dp[2] + dp[3]) % mod;
+
+			// 'e' <- 'ai'
+			tmp[1] = (dp[0] + dp[2]) % mod;
+
+			// 'o' <- 'i'
+			tmp[3] = dp[2];
+
+			// 'i' <- 'eo'
+			dp[2] = (dp[1] + dp[3]) % mod;
+
+			dp[0] = tmp[0];
+			dp[1] = tmp[1];
+			dp[3] = tmp[3];
+		}
+		return (((dp[1] + dp[2]) % mod + (dp[3] + dp[4]) % mod) % mod + dp[0]) % mod;
+	}
+};
+
+int main() {
+	Solution solver;
+	std::cout << solver.countVowelPermutation(20000);
+	return 0;
+}
\ No newline at end of file
diff --git a/2201/CMakeLists.txt b/2201/CMakeLists.txt
index aefcd37..89bead9 100644
--- a/2201/CMakeLists.txt
+++ b/2201/CMakeLists.txt
@@ -3,4 +3,4 @@ PROJECT(2201)
 
 SET(CMAKE_CXX_STANDARD 23)
 
-ADD_EXECUTABLE(2201 220116.cpp)
\ No newline at end of file
+ADD_EXECUTABLE(2201 220117-CN.cpp)
\ No newline at end of file