added longestPalindrome easy leetcode solution

Author: DevDelvo

Easy/longestPalindrome.js

@@ -0,0 +1,40 @@
+// Given a string which consists of lowercase or uppercase letters,
+// find the length of the longest palindromes that can be built with those letters.
+// This is case sensitive, for example "Aa" is not considered a palindrome here.
+
+// Note:
+// Assume the length of given string will not exceed 1,010.
+// Example:
+// Input:
+// "abccccdd"
+// Output:
+// 7
+
+// Explanation:
+// One longest palindrome that can be built is "dccaccd", whose length is 7.
+
+const test1 = "abccccdd"; 
+
+const longestPalindrome = (str) => {
+    if (!str.length) return 0;
+    if (str.length === 1) return 1;
+    let longestCount = 0; 
+    const strMap = {};
+    for (const char of str) {
+        if (strMap[char]) {
+            strMap[char]++;
+        } else {
+            strMap[char] = 1;
+        }
+    }
+    for (const key in strMap) {
+        longestCount += Math.floor(strMap[key] / 2) * 2;
+        if (longestCount % 2 === 0 && strMap[key] % 2 === 1) {
+            longestCount++;
+        }
+    }
+    return longestCount;
+}
+
+console.log(longestPalindrome(test1)); // 7
+// console.log(longestPalindrome(test2)); // 7