Update README for problem 678

Author: anirudhology

README.md

@@ -124,6 +124,7 @@ LeetCode solutions in Python
 | 567        | Permutation In String                                     | [Solution](problems/sliding_window/permutation_in_string.py)                               | Medium     | Sliding Window                        |
 | 621        | Task Scheduler                                            | [Solution](problems/heap/task_scheduler.py)                                                | Medium     | Heap, Queue                           |
 | 647        | Palindromic Substrings                                    | [Solution](problems/dynamic_programming/palindromic_substrings.py)                         | Medium     | Dynamic Programming, Two Pointers     |
+| 678        | Valid Parenthesis String                                  | [Solution](problems/greedy/valid_parenthesis_string.py)                                    | Medium     | Greedy                                |
 | 684        | Redundant Connection                                      | [Solution](problems/union_find/redundant_connection.py)                                    | Medium     | Union Find, Graph                     |
 | 695        | Max Area Of Island                                        | [Solution](problems/dfs/max_area_of_island.py)                                             | Medium     | DFS                                   |
 | 703        | Kth Largest Element In A Stream                           | [Solution](problems/heap/kth_largest_element_in_a_stream.py)                               | Easy       | Heap                                  |