3. Longest Substring Without Repeating Characters

Approach: Sliding Window with a Set holding the unique characters

public int lengthOfLongestSubstring(String s) {
  int max = 0, left = 0, right = 0;
  Set<Character> charsInWindow = new HashSet<>();

  while (right < s.length()) {
    if (charsInWindow.contains(s.charAt(right))) {
      charsInWindow.remove(s.charAt(left));
      left++;
    } else {
      charsInWindow.add(s.charAt(right));
      right++;
      max = Math.max(max, charsInWindow.size());
    }
  }

  return max;
}