# Time: O(N)
# Space: O(N)
class Solution:
    def lengthOfLongestSubstring(self, s: str) -> int:
        '''
        Sliding-window approach
        '''
        
        # Keep a track of unique chars we've seen so far in the substring
        # we are looking at
        seen = set()
        start, end = 0, 0
        maxl = 0
        
        while start <= end < len(s):
            # Our current substring is denoted by their bounds [start, end]
            # If we see a char in `end` which is already seen by us, we need
            # to keep moving `start` until we are left with a new character
            while s[end] in seen:
                # Since `start` is moving up, we need to remove existing
                # `start` char from the set if it exists
                seen.remove(s[start])
                start += 1
            
            seen.add(s[end])
            maxl = max(maxl, (end - start) + 1)
            end += 1
        
        return maxl