Update 5. Longest Palindromic Substring.md

Author: ps0305

001-100/5. Longest Palindromic Substring.md

@@ -65,3 +65,38 @@ var expandAroundCenter = function (s, left, right) {
 
 * Time complexity : O(n^2).
 * Space complexity : O(1).
+
+
+```js
+/**
+ * @param {string} s
+ * @return {string}
+ */
+var longestPalindrome = function(s) {
+    let startIndex = 0;
+    let maxLength = 1;
+    
+    function expandAroundCenter(left, right) {
+        while (left >=0 && right < s.length && s[left] === s[right]) {
+            const currentPalLength = right - left + 1;
+            if (currentPalLength > maxLength) {
+                maxLength = currentPalLength;
+                startIndex = left;
+            }
+            left -= 1;
+            right += 1;
+        }
+    }
+    
+    for (let i = 0; i < s.length; i++) {
+            expandAroundCenter(i-1, i+1);
+            expandAroundCenter(i, i+1);
+        }
+    
+    return s.slice(startIndex, startIndex + maxLength)
+};
+```
+**Complexity:**
+
+* Time complexity : O(n^2).
+* Space complexity : O(1).