Pointer Manipulation Target: 15s
Set-based window tracks unique characters. Shrink left when a duplicate enters.
seen = set()
left = 0
best = 0
for right in range(len(s)):
while s[right] in seen:
seen.remove(s[left])
left += 1
seen.add(s[right])
best = max(best, right - left + 1)Type it from memory. Go.