We use a sliding window with two pointers (left and right) and a hash map to store the last seen index of each character.

1. l = 0, res = 0.
2. char_map = {}.
3. For r from 0 to len(s)-1:
   • If s[r] is in char_map:
     • Update l = max(l, char_map[s[r]] + 1).
   • Update res = max(res, r - l + 1).
   • char_map[s[r]] = r.
4. Return res.

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

def length_of_longest_substring(s):
    l = 0
    res = 0
    char_map = {}
    
    for r in range(len(s)):
        if s[r] in char_map:
            l = max(l, char_map[s[r]] + 1)
        res = max(res, r - l + 1)
        char_map[s[r]] = r
        
    return res