Add solution #921

Author: JoshCrozier

README.md

@@ -730,6 +730,7 @@
 918|[Maximum Sum Circular Subarray](./solutions/0918-maximum-sum-circular-subarray.js)|Medium|
 919|[Complete Binary Tree Inserter](./solutions/0919-complete-binary-tree-inserter.js)|Medium|
 920|[Number of Music Playlists](./solutions/0920-number-of-music-playlists.js)|Hard|
+921|[Minimum Add to Make Parentheses Valid](./solutions/0921-minimum-add-to-make-parentheses-valid.js)|Medium|
 922|[Sort Array By Parity II](./solutions/0922-sort-array-by-parity-ii.js)|Easy|
 925|[Long Pressed Name](./solutions/0925-long-pressed-name.js)|Easy|
 926|[Flip String to Monotone Increasing](./solutions/0926-flip-string-to-monotone-increasing.js)|Medium|

solutions/0921-minimum-add-to-make-parentheses-valid.js

@@ -0,0 +1,38 @@
+/**
+ * 921. Minimum Add to Make Parentheses Valid
+ * https://leetcode.com/problems/minimum-add-to-make-parentheses-valid/
+ * Difficulty: Medium
+ *
+ * A parentheses string is valid if and only if:
+ * - It is the empty string,
+ * - It can be written as AB (A concatenated with B), where A and B are valid strings, or
+ * - It can be written as (A), where A is a valid string.
+ *
+ * You are given a parentheses string s. In one move, you can insert a parenthesis at any position
+ * of the string.
+ * - For example, if s = "()))", you can insert an opening parenthesis to be "(()))" or a closing
+ *   parenthesis to be "())))".
+ *
+ * Return the minimum number of moves required to make s valid.
+ */
+
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var minAddToMakeValid = function(s) {
+  let openCount = 0;
+  let unmatchedClose = 0;
+
+  for (const character of s) {
+    if (character === '(') {
+      openCount++;
+    } else if (openCount > 0) {
+      openCount--;
+    } else {
+      unmatchedClose++;
+    }
+  }
+
+  return openCount + unmatchedClose;
+};