Find the longest substring without repeating characters.

Focus on breaking down the problem into manageable steps. Start by clarifying the definition of a substring and what it means for characters to repeat. Discuss potential data structures you might use, such as a hash map or a sliding window technique, to efficiently track characters and their indices. Be prepared to explain your thought process as you outline your approach, including time and space complexity considerations. Finally, walk through a sample input to demonstrate how your solution would work in practice.