add longest-palindrome script.

Author: everthis

409-longest-palindrome.js

@@ -0,0 +1,29 @@
+/**
+ * @param {string} s
+ * @return {number}
+ */
+const longestPalindrome = function(s) {
+  const hash = {};
+  let c;
+  for (let i = 0; i < s.length; i++) {
+    c = s.charAt(i);
+    if (hash.hasOwnProperty(c)) {
+      hash[c] += 1;
+    } else {
+      hash[c] = 1;
+    }
+  }
+  let res = 0;
+  let val;
+  for (let k in hash) {
+    if (hash.hasOwnProperty(k)) {
+      val = hash[k];
+      res += Math.floor(val / 2) * 2;
+      if (res % 2 === 0 && val % 2 === 1) {
+        res += 1;
+      }
+    }
+  }
+
+  return res;
+};