Jinwook Jun contributed to the DaleStudy/leetcode-study repository by developing a hash map-based duplicate detection feature for arrays, focusing on improving runtime efficiency in common workflows. Using C++ and leveraging data structures and algorithm development skills, Jinwook implemented a solution that accelerates duplicate checks through efficient hash map lookups. Additionally, Jinwook addressed a code quality issue by ensuring files end with a trailing newline, aligning the project with coding standards and enhancing maintainability. The work demonstrated attention to both performance and code hygiene, resulting in more reliable code and reinforcing best practices within the repository during the month-long contribution period.