Add solution #1220

Author: JoshCrozier

README.md

@@ -1,4 +1,4 @@
-# 1,183 LeetCode solutions in JavaScript
+# 1,184 LeetCode solutions in JavaScript
 
 [https://leetcodejavascript.com](https://leetcodejavascript.com)
 
@@ -937,6 +937,7 @@
 1217|[Minimum Cost to Move Chips to The Same Position](./solutions/1217-minimum-cost-to-move-chips-to-the-same-position.js)|Easy|
 1218|[Longest Arithmetic Subsequence of Given Difference](./solutions/1218-longest-arithmetic-subsequence-of-given-difference.js)|Medium|
 1219|[Path with Maximum Gold](./solutions/1219-path-with-maximum-gold.js)|Medium|
+1220|[Count Vowels Permutation](./solutions/1220-count-vowels-permutation.js)|Hard|
 1232|[Check If It Is a Straight Line](./solutions/1232-check-if-it-is-a-straight-line.js)|Easy|
 1233|[Remove Sub-Folders from the Filesystem](./solutions/1233-remove-sub-folders-from-the-filesystem.js)|Medium|
 1249|[Minimum Remove to Make Valid Parentheses](./solutions/1249-minimum-remove-to-make-valid-parentheses.js)|Medium|

solutions/1220-count-vowels-permutation.js

@@ -0,0 +1,47 @@
+/**
+ * 1220. Count Vowels Permutation
+ * https://leetcode.com/problems/count-vowels-permutation/
+ * Difficulty: Hard
+ *
+ * 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.
+ */
+
+/**
+ * @param {number} n
+ * @return {number}
+ */
+var countVowelPermutation = function(n) {
+  const MOD = 1000000007n;
+  let aCount = 1n;
+  let eCount = 1n;
+  let iCount = 1n;
+  let oCount = 1n;
+  let uCount = 1n;
+
+  for (let length = 2; length <= n; length++) {
+    const prevACount = aCount;
+    const prevECount = eCount;
+    const prevICount = iCount;
+    const prevOCount = oCount;
+    const prevUCount = uCount;
+
+    aCount = (prevECount + prevICount + prevUCount) % MOD;
+    eCount = (prevACount + prevICount) % MOD;
+    iCount = (prevECount + prevOCount) % MOD;
+    oCount = (prevICount) % MOD;
+    uCount = (prevICount + prevOCount) % MOD;
+  }
+
+  const total = aCount + eCount + iCount + oCount + uCount;
+
+  return Number(total % MOD);
+};