how about this solution, basically once we encounter a duplicate then just clear the set

int windowStart = 0, maxLength = 0; Set cache = new HashSet(); for (int windowEnd = 0; windowEnd < str.length(); windowEnd++) { char c = str.charAt(windowEnd); if (cache.contains(c)) { cache.clear(); windowStart = windowEnd; } cache.add(c); maxLength = Math.max(maxLength, windowEnd - windowStart + 1); }

return maxLength;