Question d’entretien chez Fidelity Investments

Question: Given a string, write a function to find the longest substring without repeating characters. The function should return the length of this substring. Example: Input: "abcabcbb" Output: 3 (Explanation: The answer is "abc" with the length of 3.) Constraints: Assume the input string only contains ASCII characters. Aim for an optimal solution in terms of time complexity.