APCS/04 ListNode/02 ListLabReverse/ListLabReverse.java

// Name: B6-24
// Date: 11/8/19

/*****************************************
Demonstrates many ways to reverse a list made of ListNodes.
******************************************/
public class ListLabReverse
{
   public static void main(String[] args)
   {
      ListNode head = new ListNode("hello", null);
      head = new ListNode("foo", head);
      head = new ListNode("boo", head);
      head = new ListNode("nonsense", head);
      head = new ListNode("computer",
         new ListNode("science",
         new ListNode("java",
         new ListNode("coffee", head))));
         
      System.out.print("print the original: \t\t\t\t");
      print(head);
         
      System.out.print("recur and print: \t\t\t\t\t");
      recurAndPrint(head);
      
      System.out.println();
      System.out.print("original is unchanged: \t\t\t\t");
      print(head);
      
      System.out.print("reverse by building a new list: \t");
      head = reverseBuildNew(head);
      print(head);      
      	
      System.out.print("iterate with 3 pointers:\t\t\t");
      head = iterateThreePointers(head);
      print(head);
      	
      System.out.print("recur with 2 pointers: \t\t\t\t");
      head = recurTwoPointers(null, head);
      print(head);
              
      System.out.print("recur with pointers and append: \t");
      head = recurPointersAppend(head);
      print(head);
      	
      System.out.print("Mind Bender reverse:\t\t\t\t\t");
      head = mindBender(head);
      print(head);
   }
   
   public static void print(ListNode head)
   {
      System.out.print("[");
      while(head != null)
      {
         System.out.print(head.getValue());
         head = head.getNext();
         if(head != null)
            System.out.print(", ");
      }
      
      System.out.println("]");
   }
   
	/*********************************************
	1. This approach doesn't actually reverse the list.  It only prints
	the list in reverse order.  recurAndPrint() prints the square 
	brackets and calls helper().  helper() is recursive.
	********************************************************/
   public static void recurAndPrint(ListNode h)
   {
      System.out.print("[");
      helper(h);
      System.out.print("]");
   
   }
   
   private static void helper(ListNode p)
   {
      if (p == null)
         return;
      if (p.getNext() == null)
         System.out.print(p.getValue());
      else {
         helper(p.getNext());
         System.out.print(", " + p.getValue());
      }
   
   }
   
   /*********************************************
	2. This iterative method (for or while) produces a copy of the 
	reversed list.	 For each node going forward, make a new node and 
	link it to the list.	The list will naturally be in reverse. 
	***********************************************************/
   public static ListNode reverseBuildNew(ListNode head)
   {
      ListNode reverse = null;
      for (ListNode pointer = head; pointer != null; pointer = pointer.getNext())
            reverse = new ListNode(pointer.getValue(), reverse);
      
      return reverse;
   }

   /*******************************************
	3a. This iterative method (while) uses 3 pointers to reverse 
	the list in place.   The two local pointers are called
	prev and next.
   ********************************************************/
   public static ListNode iterateThreePointers(ListNode head)
   {
      if(head == null) 
         return null;
      ListNode prev = null, next = head.getNext();
      /*  enter your code here   */
      while (head != null) {
         next = head.getNext();
         head.setNext(prev);
         prev = head;
         head = next;
      }
   
      return prev;
   }
   	
	/**************************************************
	3b. This recursive method uses two pointers as arguments to reverse 
	the list in place. Each level creates and uses a third pointer, called "next".
   ********************************************************/
   public static ListNode recurTwoPointers(ListNode prev, ListNode head)
   {
      if (head == null) {
         return prev;
      } else {
         ListNode next = head.getNext();
         head.setNext(prev);
         return recurTwoPointers(head, next);
      }
   }
    
   
   /**********************************************
	3c. On each recursive level, find pointerToLast() and 
	nextToLast(). Make a new last. On way back, append() 
	one level to the other. 
	********************************************************/
   public static ListNode recurPointersAppend(ListNode head)
   {
      if (head == null || head.getNext() == null)
         return head;
         
      ListNode last = pointerToLast(head);
      ListNode nextLast = nextToLast(head);
      nextLast.setNext(null);
      
      return append(last, recurPointersAppend(head));
   }
   
   private static ListNode pointerToLast(ListNode head)
   {
      if (head == null)
         return null;
      
      ListNode pointer = head;   
      while (pointer.getNext() != null)
         pointer = pointer.getNext();
      
      return pointer;
   }
   
   private static ListNode nextToLast(ListNode head)
   {
      if (head == null)
         return null;
      
      ListNode pointer = head;   
      while (pointer.getNext().getNext() != null)
         pointer = pointer.getNext();
      
      return pointer;
   }
   
   private static ListNode append(ListNode h1, ListNode h2)
   {
      if (h1 == null)
         return null;
         
      ListNode temp = pointerToLast(h1);
      temp.setNext(h2);
      return temp;
   }

   /**********************************************
   3d. This difficult method reverses the list in place, using one
   local pointer. Start with pointerToLast(). The helper method
   is recursive.
	********************************************************/
   public static ListNode mindBender(ListNode head)
   {
      ListNode temp = pointerToLast(head);
      mindBenderHelper(head);
      head.setNext(null);
      return temp;
   }
   
   public static void mindBenderHelper(ListNode head)
   {
      if (head == null || head.getNext() == null)
         return;
      
      mindBenderHelper(head.getNext());
      ListNode rest = head.getNext();
      head.setNext(null); 
      rest.setNext(head);
   }
}

/********************************************
 print the original: 				[computer, science, java, coffee, nonsense, boo, foo, hello]
 recur and print: 					[hello, foo, boo, nonsense, coffee, java, science, computer]
 
 original is unchanged: 				[computer, science, java, coffee, nonsense, boo, foo, hello]
 reverse by building a new list: 	[hello, foo, boo, nonsense, coffee, java, science, computer]
 iterate with 3 pointers:			   [computer, science, java, coffee, nonsense, boo, foo, hello]
 recur with 2 pointers: 				[hello, foo, boo, nonsense, coffee, java, science, computer]
 recur with pointers and append: 	[computer, science, java, coffee, nonsense, boo, foo, hello]
 Mind Bender reverse:					[hello, foo, boo, nonsense, coffee, java, science, computer]

**************************************/