Add solution #3

Author: JoshCrozier

README.md

@@ -8,6 +8,7 @@
 |:---|:---|:---|
 1|[Two Sum](./0001-two-sum.js)|Easy|
 2|[Add Two Numbers](./0002-add-two-numbers.js)|Medium|
+3|[Longest Substring Without Repeating Characters](./0003-longest-substring-without-repeating-characters.js)|Medium|
 4|[Median of Two Sorted Arrays](./0004-median-of-two-sorted-arrays.js)|Hard|
 7|[Reverse Integer](./0007-reverse-integer.js)|Easy|
 27|[Remove Element](./0027-remove-element.js)|Easy|

solutions/0003-longest-substring-without-repeating-characters.js

@@ -0,0 +1,22 @@
+/**
+ * 3. Longest Substring Without Repeating Characters
+ * https://leetcode.com/problems/longest-substring-without-repeating-characters/
+ * Difficulty: Medium
+ *
+ * Given a string `s`, find the length of the longest substring without repeating characters.
+ */
+
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var lengthOfLongestSubstring = function(s) {
+  const map = {};
+  let offset = 0;
+
+  return s.split('').reduce((max, value, i) => {
+    offset = map[value] >= offset ? map[value] + 1 : offset;
+    map[value] = i;
+    return Math.max(max, i - offset + 1);
+  }, 0);
+};