19. Remove Nth Node From End of List

19. Remove Nth Node From End of List.swift

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.next = nil
 *     }
 * }
 */
class Solution {
    
    func removeNthFromEnd(_ head: ListNode?, _ n: Int) -> ListNode? {
        
        var p1 = head
        var p2 = p1
        var prev = p1
        
        if n == 1 && p1?.next == nil {
            return nil
        }
        
        for _ in 0 ..< n {
            p1 = p1?.next
        }
        if p1 == nil {
            return head?.next
        }
        while p1 != nil {
            p1 = p1?.next
            prev = p2
            p2 = p2?.next
            
            if p1 == nil {
                prev?.next = p2?.next
            }
        }
        
        return head
    }
}