LeetCode-in-Swift

19. Remove Nth Node From End of List

Medium

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

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

Follow up: Could you do this in one pass?

To solve the Remove Nth Node From End of List problem in Swift with a Solution class, we’ll follow these steps:

1. Define a ListNode class representing the nodes of the linked list.
2. Define a Solution class with a method named removeNthFromEnd that takes the head of the linked list and an integer n as input and returns the head of the modified list.
3. Create two pointers, fast and slow, and initialize them to point to the head of the list.
4. Move the fast pointer n steps forward in the list.
5. If the fast pointer reaches the end of the list (fast == null), it means that n is equal to the length of the list. In this case, remove the head node by returning head.next.
6. Move both fast and slow pointers simultaneously until the fast pointer reaches the end of the list.
7. At this point, the slow pointer will be pointing to the node just before the node to be removed.
8. Remove the nth node by updating the next reference of the node pointed to by the slow pointer to skip the nth node.
9. Return the head of the modified list.

Here’s the implementation:

class Solution {
    func removeNthFromEnd(_ head: ListNode?, _ n: Int) -> ListNode? {        
        guard ((head?.next) != nil), n > 0 else {return nil}        
        var count = 0
        var current = head        
        while let currentNode = current{
            count += 1
            current = currentNode.next
        }        
        current = head
        count = count - n + 1      
        var prev: ListNode?        
        while let currentNode = current {            
            count -= 1            
            if count == 0 {            
                if prev == nil {
                    current = current?.next
                    return current
                } else {
                    prev?.next = current?.next
                }
                break
            }            
            prev = current
            current = current?.next            
        }        
        return head        
    }
}

This implementation provides a solution to the Remove Nth Node From End of List problem in Swift.

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