Problem2-1-with-buffer.java

class Node {
  Node next = null;
  char data;
  
  public Node(char c) {
    data = c;
  }
}

// FOLLOW UP -> FOLW UP
public Node removeDuplicates(Node head) {
  if (head == null) 
    return null;
    
  Node current = head;
  Node prev = head;
    
  // Ask if ASCII
  boolean[] seen = new boolean[256];

  while (current != null) {
    if (! seen[current.data]) {
      seen[current.data] = true;
    }
    else {
      prev.next = current.next; // skip current
    }
    prev = current;
    current = current.next;
  }
  
  return head;
}