0019.删除链表的倒数第N个结点.md

19.删除链表的倒数第N个结点

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode fast = dummy;
        ListNode slow = dummy;

        for (int i = 0; i <= n; i++) {
            fast = fast.next;
        }

        while (fast != null) {
            fast = fast.next;
            slow = slow.next;
        }

        slow.next = slow.next.next;

        return dummy.next;  
    }
}

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {

        ListNode dummy = new ListNode(-1);
        dummy.next = head;

        int k = 0;
        ListNode p = dummy;
        while (p != null) {
            k++;
            p = p.next;
        }

        p = dummy;
        for (int i = 0; i < k - n - 1; i++) {
            p = p.next;
        }

        p.next = p.next.next;
        return dummy.next; 
    }
}