Add 0020-valid-parentheses to topics.

Author: shimhun99

README.md

@@ -45,6 +45,7 @@
 ## String
 | Problem Name | Difficulty |
 | ------- | ------- |
+| [0020-valid-parentheses](https://github.com/SHIMHUN/Algorithm/tree/main/LeetCode/Easy/0020-valid-parentheses/) | Easy |
 | [0049-group-anagrams](https://github.com/SHIMHUN/Algorithm/tree/main/LeetCode/Medium/0049-group-anagrams/) | Medium |
 | [0125-valid-palindrome](https://github.com/SHIMHUN/Algorithm/tree/main/LeetCode/Easy/0125-valid-palindrome/) | Easy |
 | [0242-valid-anagram](https://github.com/SHIMHUN/Algorithm/tree/main/LeetCode/Easy/0242-valid-anagram/) | Easy |
@@ -69,4 +70,12 @@
 | ------- | ------- |
 | [0049-group-anagrams](https://github.com/SHIMHUN/Algorithm/tree/main/LeetCode/Medium/0049-group-anagrams/) | Medium |
 | [0242-valid-anagram](https://github.com/SHIMHUN/Algorithm/tree/main/LeetCode/Easy/0242-valid-anagram/) | Easy |
+## Stack
+| Problem Name | Difficulty |
+| ------- | ------- |
+| [0020-valid-parentheses](https://github.com/SHIMHUN/Algorithm/tree/main/LeetCode/Easy/0020-valid-parentheses/) | Easy |
+## Bracket Sequences
+| Problem Name | Difficulty |
+| ------- | ------- |
+| [0020-valid-parentheses](https://github.com/SHIMHUN/Algorithm/tree/main/LeetCode/Easy/0020-valid-parentheses/) | Easy |
 <!---LeetCode Topics End-->