从链接列表的末尾删除第K个节点

Question

Everything is working except for when I want to remove the first node in the list if (kthToLast == 0) head = head.next; . I do not know why it does not remove the first element since head = head.next; should work.

Thanks.

// 2.2 Implement an algorithm to find the kth to last element of a singly linked list.

public class RemoveKthToLast {

    static Node head;
    static int count = 0;

    public static class Node {
        int data;
        Node next;

        Node(int data) {
            this.data = data;
            next = null;
            count++;
        }
    }

    public Node removeKthToLast(Node head, int n) {
        if (head == null || n < 1)
            System.out.println("invalid");;

        int count = 0;
        Node temp = head;
        while (temp != null) { // count number of nodes in list
            temp = temp.next;
            count++;
        }

        if (n > count)
            System.out.println("n is greater than length of list");;

        int kthToLast = count - n;

        // remove first node 
        if(kthToLast == 0) 
             head = head.next;

        for (int i = 0; i < kthToLast - 1; i++) 
            head = head.next;

        head.next = head.next.next;
        return head;
    }

    // prints out contents of linked-list
    public void toString(Node node) {
        while (node != null) {
            System.out.print(node.data + " ");
            node = node.next;
        }
    }

    public static void main(String args[]) {

        RemoveKthToLast list = new RemoveKthToLast();

        list.head = new Node(1);
        list.head.next = new Node(2);
        list.head.next.next = new Node(3);
        list.head.next.next.next = new Node(4);

        list.removeKthToLast(head, 0);
        list.toString(head);
    }
}

Answer 1

If the only problem is that the first node isn't removed then try this

// remove first node 
if(kthToLast == 0) {
     head = head.next;
     return head;
}

Answer 2

The problem is that even when you're trying to delete the first node in the linked list, you still proceed with the rest of the method, which should only be executed when trying to remove nodes other than the first node. For example, let's say you're trying to remove the head. Yes, the head field gets updated to the 2nd node in the list. However, the method doesn't end there and executes the line head.next = head.next.next which will incorrectly delete the 3rd node in the linked list. Therefore, the solution is to basically end the method once the head field is reset.

public Node removeKthToLast(Node head, int n) {
    if (head == null || n < 1)
        System.out.println("invalid");;

    int count = 0;
    Node temp = head;
    while (temp != null) { // count number of nodes in list
        temp = temp.next;
        count++;
    }

    if (n > count)
        System.out.println("n is greater than length of list");;

    int kthToLast = count - n;

    // remove first node 
    if(kthToLast == 0){
         head = head.next;
         return head;//end the method here when removing the 1st node
    }
    for (int i = 0; i < kthToLast - 1; i++) 
        head = head.next;

    head.next = head.next.next;
    return head;
    }

Also, make sure to update your list's head node in the main.

list.head = list.removeKthToLast(head,4);

Answer 3

Just a suggestion: you could use a recursive function here and your code will be astoundly simple:

public Node removeKthFromLast(Node head, int k) {
    int n;
    Node p = head;
    for (n=0;head != null;n++) {
      p = head.next;
    }
    return removeKthFromLastAux(head, n-k);
}

private Node removeKthFromLastAux(Node head, int ik) {
    if (head == null) {
        return null;
    }
    if (ik == 0) {
        return head.next;
    }
    head.next = removeKthFromLastAux(head, ik-1);
    return head;
}