20. Valid Parentheses https://leetcode.com/problems/valid-parentheses
921. Minimum Add to Make Parentheses Valid https://leetcode.com/problems/minimum-add-to-make-parentheses-valid
1541. Minimum Insertions to Balance a Parentheses String https://leetcode.com/problems/minimum-insertions-to-balance-a-parentheses-string
Prerequisite Knowledge
Before reading this article, you should first learn:
