def sliding_window(s):
    left = 0
    window_state = {} # Or set(), or integer sum
    best_len = 0
    
    # 1. Expand (Right pointer moves)
    for right in range(len(s)):
        char = s[right]
        window_state[char] = window_state.get(char, 0) + 1
        
        # 2. Shrink (While invalid)
        # Example: If we have duplicates
        while window_state[char] > 1:
            left_char = s[left]
            window_state[left_char] -= 1
            left += 1
        
        # 3. Update Result (After fixing the window)
        best_len = max(best_len, right - left + 1)
        
    return best_len