Given a string s, find the length of the longest substring without repeating characters. You must achieve an O(n) time complexity. ### Constraints: - 0 <= s.length <= 5 * 10^4 - s consists of English letters, digits, symbols, and spaces. ### Examples: 1. Input: s = 'abcabcbb' Output: 3 Explanation: The answer is 'abc', with the length of 3. 2. Input: s = 'bbbbb' Output: 1 Explanation: The answer is 'b', with the length of 1.