Update longest_valid_paranthesis.py

added necessary changes!

Author: AryanMankame

strings/longest_valid_paranthesis.py

@@ -23,10 +23,11 @@ def longest_valid_paranthesis(self, s: str) -> int:
             else:
                 res = max(res, preceeding_matched)
                 preceeding_matched = 0
-
     res = max(res, preceeding_matched)
 
     while stack:
         res = max(res, stack.pop())
-
     return res * 2
+if __name__ == "__main__":
+    from doctest import testmod
+    testmod()