参与本项目,贡献其他语言版本的代码,拥抱开源,让更多学习算法的小伙伴们收益!

# 19.删除链表的倒数第N个节点

力扣题目链接 (opens new window)

给你一个链表,删除链表的倒数第 n 个结点,并且返回链表的头结点。

进阶:你能尝试使用一趟扫描实现吗?

示例 1:

19.删除链表的倒数第N个节点

输入:head = [1,2,3,4,5], n = 2 输出:[1,2,3,5] 示例 2:

输入:head = [1], n = 1 输出:[] 示例 3:

输入:head = [1,2], n = 1 输出:[1]

# 思路

双指针的经典应用,如果要删除倒数第n个节点,让fast移动n步,然后让fast和slow同时移动,直到fast指向链表末尾。删掉slow所指向的节点就可以了。

思路是这样的,但要注意一些细节。

分为如下几步:

  • fast首先走n + 1步 ,为什么是n+1呢,因为只有这样同时移动的时候slow才能指向删除节点的上一个节点(方便做删除操作),如图:

  • fast和slow同时移动,直到fast指向末尾,如题:

  • 删除slow指向的下一个节点,如图:

此时不难写出如下C++代码:

class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode* dummyHead = new ListNode(0);
        dummyHead->next = head;
        ListNode* slow = dummyHead;
        ListNode* fast = dummyHead;
        while(n-- && fast != NULL) {
            fast = fast->next;
        }
        fast = fast->next; // fast再提前走一步,因为需要让slow指向删除节点的上一个节点
        while (fast != NULL) {
            fast = fast->next;
            slow = slow->next;
        }
        slow->next = slow->next->next;
        return dummyHead->next;
    }
};
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19

# 其他语言版本

java:

class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode dummy = new ListNode(-1);
        dummy.next = head;

        ListNode slow = dummy;
        ListNode fast = dummy;
        while (n-- > 0) {
            fast = fast.next;
        }
        // 记住 待删除节点slow 的上一节点
        ListNode prev = null;
        while (fast != null) {
            prev = slow;
            slow = slow.next;
            fast = fast.next;
        }
        // 上一节点的next指针绕过 待删除节点slow 直接指向slow的下一节点
        prev.next = slow.next;
        // 释放 待删除节点slow 的next指针, 这句删掉也能AC
        slow.next = null;

        return dummy.next;
    }
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25

Python:

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def removeNthFromEnd(self, head: ListNode, n: int) -> ListNode:
        head_dummy = ListNode()
        head_dummy.next = head

        slow, fast = head_dummy, head_dummy
        while(n!=0): #fast先往前走n步
            fast = fast.next
            n -= 1
        while(fast.next!=None):
            slow = slow.next
            fast = fast.next
        #fast 走到结尾后,slow的下一个节点为倒数第N个节点
        slow.next = slow.next.next #删除
        return head_dummy.next
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

Go:

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func removeNthFromEnd(head *ListNode, n int) *ListNode {
    dummyHead := &ListNode{}
    dummyHead.Next = head
    cur := head
    prev := dummyHead
    i := 1
    for cur != nil {
        cur = cur.Next
        if i > n {
            prev = prev.Next
        }
        i++
    }
    prev.Next = prev.Next.Next
    return dummyHead.Next
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23

JavaScript:

/**
 * @param {ListNode} head
 * @param {number} n
 * @return {ListNode}
 */
var removeNthFromEnd = function(head, n) {
    let ret = new ListNode(0, head),
        slow = fast = ret;
    while(n--) fast = fast.next;
    if(!fast) return ret.next;
    while (fast.next) {
        fast = fast.next; 
        slow = slow.next
    };
    slow.next = slow.next.next;
    return ret.next;
};
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

Kotlin:

fun removeNthFromEnd(head: ListNode?, n: Int): ListNode? {
    val pre = ListNode(0).apply {
        this.next = head
    }
    var fastNode: ListNode? = pre
    var slowNode: ListNode? = pre
    for (i in 0..n) {
        fastNode = fastNode?.next
    }
    while (fastNode != null) {
        slowNode = slowNode?.next
        fastNode = fastNode.next
    }
    slowNode?.next = slowNode?.next?.next
    return pre.next
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

Swift:

func removeNthFromEnd(_ head: ListNode?, _ n: Int) -> ListNode? {
    if head == nil {
        return nil
    }
    if n == 0 {
        return head
    }
    let dummyHead = ListNode(-1, head)
    var fast: ListNode? = dummyHead
    var slow: ListNode? = dummyHead
    // fast 前移 n
    for _ in 0 ..< n {
        fast = fast?.next
    }
    while fast?.next != nil {
        fast = fast?.next
        slow = slow?.next
    }
    slow?.next = slow?.next?.next
    return dummyHead.next
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21