19. 删除链表的倒数第N个节点

快慢指针

/**
 * 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) {
        // 哑结点避免边界情况
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode slow = dummy, fast = dummy;
        // 让快指针先n步
        while (n-- > 0) {
            fast = fast.next;
        }
        // 快慢指针同时往前移动，直至快指针到链表末尾 
        while (fast.next != null) {
            slow = slow.next;
            fast = fast.next;
        }
        // 删除第 n 个节点
        slow.next = slow.next.next;
        return dummy.next;
    }
}

• 时间复杂度 O(n)
• 空间复杂度 O(1)