Which three categories of Korean literature are based on their writing systems?

Korean literature is categorized into three distinct groups based on the writing systems used throughout its history:1

• Works written in Classical Chinese: For centuries, Korean scholars utilized Chinese characters to compose literature, as this was the standard for educated elites and civil service examinations until the late 19th century.1
• Works written in early transcription systems: Before the development of a native alphabet, Koreans devised systems such as idu, kugyŏl, and hyangch'al to represent Korean speech, grammatical particles, and inflectional endings using Chinese characters.1
• Works written in Hangul: Following its invention in the mid-15th century, Hangul became the national alphabet, eventually allowing for a distinct body of literature written in the Korean language.1