Update longestPalindromicSubstring.md

longestPalindromicSubstring.md

 ## 题目描述
+Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.
 
+Example:
+```
+Input: "babad"
+
+Output: "bab"
+```
+> Note: "aba" is also a valid answer.
+
+
+Example:
+```
+Input: "cbbd"
+
+Output: "bb"
+ ```
 ## 思路
 
 ## 代码
+```js
+/**
+ * @param {string} s
+ * @return {string}
+ */
+var longestPalindrome = function(s) {
+    
+};
+```