Given a string s, find the length of the longest substring without repeating characters. You must achieve an O(n) time complexity.

``` def length_of_longest_substring(s): char_set = set() left = 0 max_length = 0 for right in range(len(s)): while s[right] in char_set: char_set.remove(s[left]) left += 1 char_set.add(s[right]) max_length = max(max_length, right - left + 1) return max_length ```