## Topic description:

Input a linked list and output the kth node from the bottom of the linked list.

**Problem solving plan:**

Let the linked list start from the beginning to the end, and reduce the value of k by 1 for each step you move.

①Assuming that the linked list is 1-2-3, k=4, there is no fourth-to-last node in the linked list at all.

The nodes reached are: 1-2-3

The change of k is: 3 2 1

②Assuming that the linked list is 1-2-3, k=3, and the third node from the bottom of the linked list is 3

The nodes reached are: 1-2-3

The change of k is: 2 1 0

③ Assume that the linked list is 1-2-3, k=2, and the penultimate node of the linked list is 2

The nodes reached are: 1-2-3

The change of k is: 1 0 -1

In the above three cases, if the value of k is greater than 0, it means that the k-th node from the bottom cannot be found, and the value of k is larger than the length of the linked list; if the value of k is equal to 0, it means that the head node of the linked list is the k-th node from the bottom; if the value of k is the k-th node from the bottom Less than 0, how to deal with this situation?

First, after walking from the beginning to the end, the k value is reduced by 1 for each move, and the k value becomes kN (N is the length of the linked list), then let the current k value go through the head node again, and increase the k value by 1 for each move. , when the value of k is 0, it is the Nk node, which is the previous node of the last k-th node to be found.

**The core code is** implemented as follows:

public ListNode FindKthToTail(ListNode head,int k) { if(head == null || k < 1){ System.out.println("The linked list is empty or the value of k is less than 1!"); return null; } ListNode cur = head; while(cur != null){ k--; cur = cur.next; } if(k > 0){ System.out.println("The value of k is greater than the length of the linked list!"); return null; }else if(k == 0){ return head; }else{ cur = head; while(++k != 0){ cur = cur.next; } } return cur.next; }

**Test code:**

class LinkList{ public ListNode head; private int length; class ListNode { int val; ListNode next = null; ListNode(int val) { this.val = val; } } public LinkList(){ head = null; length = 0; } // Append the node to the end of the linked list public void appendLast(int value) { ListNode node = new ListNode(value); if(head == null){ head = node; length++; return; } ListNode cur = head;//Current node while(cur.next != null){ cur = cur.next; } cur.next = node; length++; } public void printList(){ ListNode cur = head; while(cur != null){ System.out.print(cur.val + " "); cur = cur.next; } } public ListNode FindKthToTail(ListNode head,int k) { if(head == null || k < 1){ System.out.println("The linked list is empty or the value of k is less than 1!"); return null; } ListNode cur = head; while(cur != null){ k--; cur = cur.next; } if(k > 0){ System.out.println("The value of k is greater than the length of the linked list!"); return null; }else if(k == 0){ return head; }else{ cur = head; while(++k != 0){ cur = cur.next; } } return cur.next; } } public class FindKthToTail { public static void main(String[] args) { LinkList list = new LinkList(); list.appendLast(1); list.appendLast(2); list.appendLast(3); list.appendLast(4); list.appendLast(5); list.appendLast(6); list.appendLast(7); list.appendLast(8); list.printList(); System.out.println(); LinkList.ListNode ret = list.FindKthToTail(list.head, 1); if(ret == null){ System.out.println("There is an error in the linked list or the K value is set incorrectly!"); }else{ System.out.println(ret.val); } } }result: