Add solution #38

Author: JoshCrozier

README.md

@@ -40,6 +40,7 @@
 34|[Find First and Last Position of Element in Sorted Array](./0034-find-first-and-last-position-of-element-in-sorted-array.js)|Medium|
 35|[Search Insert Position](./0035-search-insert-position.js)|Easy|
 36|[Valid Sudoku](./0036-valid-sudoku.js)|Medium|
+38|[Count and Say](./0038-count-and-say.js)|Medium|
 39|[Combination Sum](./0039-combination-sum.js)|Medium|
 40|[Combination Sum II](./0040-combination-sum-ii.js)|Medium|
 41|[First Missing Positive](./0041-first-missing-positive.js)|Hard|

solutions/0038-count-and-say.js

@@ -0,0 +1,35 @@
+/**
+ * 38. Count and Say
+ * https://leetcode.com/problems/count-and-say/
+ * Difficulty: Medium
+ *
+ * The count-and-say sequence is a sequence of digit strings defined by the recursive formula:
+ * - countAndSay(1) = "1"
+ * - countAndSay(n) is the way you would "say" the digit string from countAndSay(n-1), which is
+ *   then converted into a different digit string.
+ *
+ * To determine how you "say" a digit string, split it into the minimal number of substrings such
+ * that each substring contains exactly one unique digit. Then for each substring, say the number
+ * of digits, then say the digit. Finally, concatenate every said digit.
+ *
+ * For example, the saying and conversion for digit string "3322251":
+ * Given a positive integer n, return the nth term of the count-and-say sequence.
+ */
+
+/**
+ * @param {number} n
+ * @return {string}
+ */
+var countAndSay = function(n) {
+  let result = '1';
+
+  for (let i = 1; i < n; i++) {
+    result = result.replace(/((\d)\2*)/g, '$1—')
+                   .split('—')
+                   .map(s => s ? `${s.length}${s[0]}` : '')
+                   .join('');
+  }
+
+  return result;
+};
+