Solution2.java

// 19. Remove Nth Node From End of List
// https://leetcode.com/problems/remove-nth-node-from-end-of-list/description/
//
// 使用双指针, 对链表只遍历了一遍
// 时间复杂度: O(n)
// 空间复杂度: O(1)
public class Solution2 {

    public ListNode removeNthFromEnd(ListNode head, int n) {

        ListNode dummyHead = new ListNode(0);
        dummyHead.next = head;

        ListNode p = dummyHead;
        ListNode q = dummyHead;
        for( int i = 0 ; i < n + 1 ; i ++ ){
            assert q != null;
            q = q.next;
        }

        while(q != null){
            p = p.next;
            q = q.next;
        }

        p.next = p.next.next;

        return dummyHead.next;
    }

    public static void main(String[] args) {

        int arr[] = {1, 2, 3, 4, 5};
        ListNode head = new ListNode(arr);
        System.out.println(head);

        head = (new Solution2()).removeNthFromEnd(head, 2);
        System.out.println(head);
    }
}