Added longest palindromic substring program

string/Longest_palindromic_substring.py

@@ -0,0 +1,37 @@
+
+# Given a string s, return the longest palindromic substring in s.
+
+# longestPalindrome function
+class String(object):
+    def longestPalindrome(self, s):
+        res = ""
+        for itr in range(len(s)):
+            # odd case, like "aba"
+            tmp = self.helper(s, itr, itr)
+            if len(tmp) > len(res):
+                res = tmp
+            # even case, like "abba"
+            tmp = self.helper(s, itr, itr+1)
+            if len(tmp) > len(res):
+                res = tmp
+        return res
+
+    # get the longest palindrome, 
+    # left, right are the middle indexes   
+    # from inner to outer
+    def helper(self, s, left, right):
+        while left >= 0 and right < len(s) and s[left] == s[right]:
+            left -= 1; right += 1
+        return s[left+1:right]
+
+inp = input("Enter the string: ")
+obj = String()
+print("Longest Palindromic substring is: " + obj.longestPalindrome(inp))
+
+# TIME COMPLEXITY : O(N)
+
+# Enter the string: wonderwow
+# Longest Palindromic substring is: wow
+
+# Enter the string: AABBCDEGGGAAHHUHHB
+# Longest Palindromic substring is: HHUHH