[Sword refers to offer] The Kth node from the bottom in the linked list

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: