Add solution #20

Author: JoshCrozier

README.md

@@ -21,6 +21,7 @@
 13|[Roman to Integer](./0013-roman-to-integer.js)|Easy|
 14|[Longest Common Prefix](./0014-longest-common-prefix.js)|Easy|
 17|[Letter Combinations of a Phone Number](./0017-letter-combinations-of-a-phone-number.js)|Medium|
+20|[Valid Parentheses](./0020-valid-parentheses.js)|Easy|
 21|[Merge Two Sorted Lists](./0021-merge-two-sorted-lists.js)|Easy|
 27|[Remove Element](./0027-remove-element.js)|Easy|
 31|[Next Permutation](./0031-next-permutation.js)|Medium|

solutions/0020-valid-parentheses.js

@@ -0,0 +1,37 @@
+/**
+ * 20. Valid Parentheses
+ * https://leetcode.com/problems/valid-parentheses/
+ * Difficulty: Easy
+ *
+ * Given a string s containing just the characters '(', ')', '{', '}', '[' and ']',
+ * determine if the input string is valid.
+ *
+ * An input string is valid if:
+ * - Open brackets must be closed by the same type of brackets.
+ * - Open brackets must be closed in the correct order.
+ */
+
+/**
+ * @param {string} s
+ * @return {boolean}
+ */
+var isValid = function(s) {
+  const map = {
+      '(': ')',
+      '[': ']',
+      '{': '}'
+  }
+  const stack = [];
+
+  for (let i = 0; i < s.length; i++) {
+    if (map[s[i]]) {
+      stack.push(map[s[i]]);
+    } else {
+      if (stack.pop() !== s[i]) {
+        return false;
+      }
+    }
+  }
+
+  return stack.length === 0;
+};