package LeetCode;

public class LeetCode19 {

// 19. Remove Nth Node From End of List

// https://leetcode.com/problems/remove-nth-node-from-end-of-list/description/

// 先记录链表总长度

// 需要对链表进行两次遍历

// 时间复杂度: O(n)

// 空间复杂度: O(1)

public ListNode removeNthFromEnd(ListNode head, int n) {

ListNode dummyHead = new ListNode(0);

dummyHead.next = head;

int length = 0;

for(ListNode cur = dummyHead.next; cur != null ; cur = cur.next)

length ++;

int k = length - n;

assert k >= 0;

ListNode cur = dummyHead;

for(int i = 0 ; i < k ; i ++)

cur = cur.next;

cur.next = cur.next.next;

return dummyHead.next;

}

public ListNode removeNthFromEnd2(ListNode head, int n) {

ListNode dummyHead = new ListNode(0);

dummyHead.next = head;

ListNode p = dummyHead;

ListNode q = dummyHead;

// q是虚拟头结点

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 LeetCode19()).removeNthFromEnd2(head, 3);

System.out.println(head);

}

}