Longest Substring of Unique Characters

Just want to complain about the mission. I don't know who it's designed for, but even after I went through all the missions on this island twice, this mission is the most incomprehensible, it's like it's not from this island, it's annoying that even after explanations I can't fit it in my head. I'm not saying that it's complicated, it includes dictionaries first of all, which is not strings at all, and why it looks so complicated, maybe the mission is formulated incorrectly.... The only mission on the island that I skip even on the second go-round.