19. Remove Nth Node From End of List

Problem

Given the head of a linked list, remove the nth node from the end of the list and return its head.

Follow up: Could you do this in one pass?

Example 1:

Input: head = [1,2,3,4,5], n = 2
Output: [1,2,3,5]

Example 2:

Input: head = [1], n = 1
Output: []

Example 3:

Input: head = [1,2], n = 1
Output: [1]

Constraints:

The number of nodes in the list is sz.
1 <= sz <= 30
0 <= Node.val <= 100
1 <= n <= sz

Problem Summary

Delete the nth node from the end of the linked list.

Solution Ideas

This problem is relatively simple. First, loop once to get the total length of the linked list, then loop to the node before the node to be deleted and perform the deletion. One special case to note is that the head node may need to be deleted, which should be handled separately.

There is a particularly simple solution to this problem. Set up 2 pointers, with one pointer n positions ahead of the other. Move the 2 pointers at the same time, both moving the same distance. When one pointer reaches the end, the previous pointer is the nth node from the end.

Code

package leetcode

import (
	"github.com/halfrost/leetcode-go/structures"
)

// ListNode define
type ListNode = structures.ListNode

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */

// Solution 1
func removeNthFromEnd(head *ListNode, n int) *ListNode {
	dummyHead := &ListNode{Next: head}
	preSlow, slow, fast := dummyHead, head, head
	for fast != nil {
		if n <= 0 {
			preSlow = slow
			slow = slow.Next
		}
		n--
		fast = fast.Next
	}
	preSlow.Next = slow.Next
	return dummyHead.Next
}

// Solution 2
func removeNthFromEnd1(head *ListNode, n int) *ListNode {
	if head == nil {
		return nil
	}
	if n <= 0 {
		return head
	}
	current := head
	len := 0
	for current != nil {
		len++
		current = current.Next
	}
	if n > len {
		return head
	}
	if n == len {
		current := head
		head = head.Next
		current.Next = nil
		return head
	}
	current = head
	i := 0
	for current != nil {
		if i == len-n-1 {
			deleteNode := current.Next
			current.Next = current.Next.Next
			deleteNode.Next = nil
			break
		}
		i++
		current = current.Next
	}
	return head
}