Input a list and output the reciprocal k node in the list.

Idea 1:
Require the reciprocal K node, then we must first think of the length of the list len, after the length len, the reciprocal K node is the head node to traverse len-k nodes backwards.

1) Reference to p1 pointer node

2) traversing len-k nodes after p1 image

public class FindKthToTail {

    //Find the penultimate k node in the list
    //1. Find the length len of the list
    //2. Create a new reference to head
    //3.head traverses len-k nodes to get the reciprocal K node

    public static Node FindKthToTail(Node head,int k){
        int len = getLength(head);

        if(len < k){
            return null;
        }
        int steps = len-k;
        Node result  = head;
        for(int i = 0;i<steps;i++){
            result = result.next;
        }
        return result;

    }

    public static int getLength(Node head){
        int len = 0;
        for(Node cur = head;cur != null;cur = cur.next){
            len++;
        }
        return len;
    }

Train of thought two:
1. Create two nodes so that the first reference traverses k, then the second one traverses back together.
2. When the first reference is empty, the second refers to the penultimate k node

public class FindKthToTail {

    //Find the penultimate k node in the list
    //1. Create two nodes so that the first reference traverses k, then the second one traverses back together.
    //2. When the first reference is empty, the second refers to the penultimate k node
    public static Node findKthToTail(Node head,int k){
        Node p1 = head;
        Node p2 = head;

        for(int i = 0;i<k;i++){
            if(p1 == null){
                return null;
            }
            p1 = p1.next;
        }
        while(p1 != null){
            p1 = p1.next;
            p2 = p2.next;
        }
        return p2;

    }

    public static void main(String[] args) {
        Node head = new Node(1);
        head.next = new Node(3);
        head.next.next = new Node(4);
        head.next.next.next = new Node(9);
        head.next.next.next.next = new Node(12);
        System.out.println(findKthToTail(head,2).val);
    }
}