Cover image for Interview Classic 150 Questions P19 Remove Nth Node From End of List

Interview Classic 150 Questions P19 Remove Nth Node From End of List

(Updated: )
algorithm
, , , ,
Timeline

timeline

2025-11-20

init

linked list

Problem:

P19 Delete the nth from the end of the linked list N node

https://leetcode.cn/problems/remove-nth-node-from-end-of-list/description/?envType=study-plan-v2&envId=top-interview-150

Fast and slow pointers, pay attention to special case where the node to delete is the head node

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
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
struct ListNode {
	int val;
	ListNode *next;
	ListNode()
		: val(0)
		, next(nullptr)
	{
	}
	ListNode(int x)
		: val(x)
		, next(nullptr)
	{
	}
	ListNode(int x, ListNode *next)
		: val(x)
		, next(next)
	{
	}
};

class Solution {
    public:
	ListNode *removeNthFromEnd(ListNode *head, int n)
	{
		//Two pointers, when l starts from head, r starts from head+n
		ListNode *l = head;
		ListNode *r = head;
		ListNode *p;

		for (int i = 0; i < n; i++) {
			r = r->next;
		}
		if (r == nullptr) { // n == sz
			p = head;

			head = head->next;
			delete p;
			return head;
		}

		while (r->next != nullptr) {
			r = r->next;
			l = l->next;
		}
		p = l->next;
		l->next = p->next;
		delete p;
		return head;
	}
};

leetcode hot 100 rewrite

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
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
struct ListNode {
        int val;
        ListNode *next;
        ListNode()
                : val(0)
                , next(nullptr)
        {
        }
        ListNode(int x)
                : val(x)
                , next(nullptr)
        {
        }
        ListNode(int x, ListNode *next)
                : val(x)
                , next(next)
        {
        }
};

class Solution {
    public:
        ListNode *removeNthFromEnd(ListNode *head, int n)
        {
                ListNode *virtual_head = new ListNode(0, head);
                ListNode *p = virtual_head, *q;
                int i, nr_node = 0, tpos;

                while (p->next != nullptr) {
                        p = p->next;
                        nr_node++;
                }
                tpos = nr_node - n;

                p = virtual_head;
                for (i = 0; i < tpos; i++) {
                        p = p->next;
                }
                // p->next is target
                q = p->next;
                p->next = q == nullptr ? nullptr : q->next;

                head = virtual_head->next;
                delete virtual_head;

                return head;
        }
};