UniqueSubstring

public int uniqueLength(String str) {
  char[] s = str.toCharArray();
  Set<Character> showed = new HashSet<Character>();

  int front = 0;
  int back = 0;
  int maxLen = 0;

  // unique substring [back: front - 1]
  while (front < s.length) {
    if (!showed.contains(s[front])) {
      showed.add(s[front]);
      front++;
      maxLen = Math.max(maxLen, front - back);
    } else {
      showed.remove(s[back]);
      back++;
    }
  }
  return maxLen;
}