以相反顺序打印链接列表的递归方法

Question

I'm having trouble writing a method that should accept a reference to a generic single-linked list and creating a test program to testing my method on a list of strings (print the list both in-order and reverse-order). I have already created a single-linked list called SinglyLinkedList.

Answer 1

Well if you think about recursion, you know you're going to be doing something over and over again. In this case we want to print a node over and over, but we want a different node each time.

Our first node we print should be the last in the list. That indicates to me an excellent base case.

void printReverse(Node node) {
    if(node.next != null) { // we recurse every time unless we're on the last one
        printReverse(node.next);  // this says "do this to the next node first"
    }
    System.out.println(node.data); // we'll print out our node now
}

Consider if you had

1,2,3,4

You'd call print on the node with 1 in it. It would then say "I have a next node, print that". The 2 node also has a next node, so it defers to node 3. Node 3 still has a next node, so it defers to node 4 before printing. Node 4 is willing to print itself since it has no next node. Then it returns to where node 3 left off. Now node 3 can print and go back to where node 2 left off. It prints and goes to where node 1 left off. It prints and returns to the main function.

Answer 2

For in-order, call the output method before calling the function recurisvely.

void print(Node n)
{
    if ( n != null )
    {
        System.out.println(n.value);
        print(n.next);
    }
}

For reverse, call the function first and the output.

void print(Node n)
{
    if ( n != null )
    {        
        print(n.next);
        System.out.println(n.value);
    }
}

Answer 3

I agree 100% with the answers above, for my implementation I could not get this to work without a helper class. My answer is only to expand on the answers above, in case others get compilation errors too.

public void printReverse()
{
 printReverse(head);
}

private void printReverse(GNode<T> node)
{
  if (node.next != null) 
   {
     printReverse(node.next);
   }
  System.out.println(node.data);
}