Add solution #409

Author: JoshCrozier

README.md

@@ -326,6 +326,7 @@
 405|[Convert a Number to Hexadecimal](./0405-convert-a-number-to-hexadecimal.js)|Easy|
 406|[Queue Reconstruction by Height](./0406-queue-reconstruction-by-height.js)|Medium|
 407|[Trapping Rain Water II](./0407-trapping-rain-water-ii.js)|Hard|
+409|[Longest Palindrome](./0409-longest-palindrome.js)|Easy|
 412|[Fizz Buzz](./0412-fizz-buzz.js)|Easy|
 414|[Third Maximum Number](./0414-third-maximum-number.js)|Easy|
 415|[Add Strings](./0415-add-strings.js)|Easy|

solutions/0409-longest-palindrome.js

@@ -0,0 +1,21 @@
+/**
+ * 409. Longest Palindrome
+ * https://leetcode.com/problems/longest-palindrome/
+ * Difficulty: Easy
+ *
+ * Given a string s which consists of lowercase or uppercase letters, return the length
+ * of the longest palindrome that can be built with those letters.
+ *
+ * Letters are case sensitive, for example, "Aa" is not considered a palindrome.
+ */
+
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var longestPalindrome = function(s) {
+  const set = new Set();
+  return [...s].reduce((count, c) => {
+    return set.delete(c) ? count + 2 : (set.add(c), count);
+  }, 0) + (set.size > 0 ? 1 : 0);
+};