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LeetCode 921 · Solution & editorial
A parentheses string is valid if and only if:
Minimum Add to Make Parentheses Valid is a Stack Patterns problem, in the Valid Parentheses Matching subpattern. The pattern covers parentheses matching, monotonic stack, histogram, and expression evaluation
Master parentheses validation and balancing using stack. Handle multiple bracket types and minimum additions.
The reference solution runs in O(n) time and O(1) space.
Minimum Add to Make Parentheses Valid has been reported in interviews at Facebook, Visa.
The full editorial is part of Pro
It explains the core insight, the complexity analysis and the common mistakes, with a worked solution.
Minimum Add to Make Parentheses Valid is a Stack Patterns problem, in the Valid Parentheses Matching subpattern. Once the approach above makes sense, write it yourself and then work through the neighbouring problems that reuse it.