remove-nth-node-from-end-of-list

/**
 * 给定一个链表，删除链表的倒数第n个节点并返回链表的头指针
 * 例如，
 *    给出的链表为:1->2->3->4->5, n= 2.↵↵
 *    删除了链表的倒数第n个节点之后,链表变为1->2->3->5.
 * 备注：
 * 题目保证n一定是合法的
 * 请尝试只用一步操作完成该功能
 */

public class Main58 {
    public static void main(String[] args) {
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        head.next.next.next = new ListNode(4);
        head.next.next.next.next = new ListNode(5);
        System.out.println(Main58.removeNthFromEnd(head, 2).val);
    }

    public static class ListNode {
        int val;
        ListNode next;
        ListNode(int x) {
            val = x;
            next = null;
        }
    }

    public static ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode first = head;
        ListNode second = head;
        while (n-- > 0) {
            first = first.next;
        }
        ListNode pre = null;
        while (first.next != null) {
            pre = second;
            second = second.next;
            first = first.next;
        }
        if (pre != null) {
            pre.next = second.next;
        }else{
            head = head.next;
        }
        return head;
    }
}