We use the Sliding Window technique with two pointers and a hash map (or set) to track the indices of characters seen so far.

1. start = 0, max_len = 0.
2. used = {} (map char to last seen index).
3. For end from 0 to len(s) - 1:
   • If s[end] is in used and used[s[end]] >= start:
     • Move start to used[s[end]] + 1.
   • Update max_len = max(max_len, end - start + 1).
   • Update used[s[end]] = end.
4. Return max_len.

• Time Complexity: O(N).
• Space Complexity: O(min(M, N)), where M is the size of the charset.

def length_of_longest_substring(s):
    used = {}
    max_len = start = 0
    
    for i, char in enumerate(s):
        if char in used and start <= used[char]:
            start = used[char] + 1
        else:
            max_len = max(max_len, i - start + 1)
        used[char] = i
        
    return max_len