Given a string S, find the longest palindromic substring in S. Substring of string S: S[ i . . . . j ] where 0 ≤ i ≤ j < len(S). Palindrome string: A string which reads the same backwards. More formally, S is palindrome if reverse(S) = S. Incase of conflict, return the substring which occurs first ( with the least starting index).

Example 1:

Input:
S = "aaaabbaa"
Output: aabbaa
Explanation: The longest Palindromic
substring is "aabbaa".

Example 2:

Input: 
S = "abc"
Output: a
Explanation: "a", "b" and "c" are the 
longest palindromes with same length.
The result is the one with the least
starting index.

Your Task:
You don't need to read input or print anything. Your task is to complete the function longestPalin() which takes the string S as input and returns the longest palindromic substring of S.

Expected Time Complexity: O(|S|²).
Expected Auxiliary Space: O(1).

Constraints:
1 ≤ |S| ≤ 10³