What is a Linked List?

A linked list is a common data structure that is similar to an array, but its order is based on pointers to the next element in memory instead of using physical placement (indices).

A linked list has two main components:

  1. ListNode class: This class has a val property that represents the value and a next property that represents a pointer to the next element in memory.
  2. LinkedList class: This class stores a collection of ListNode elements and provides operations like add to tail and add to head.
    • The add to tail operation takes O(n) time because it needs to iterate through the list to find the last element.
    • The add to head operation takes O(1) time because it only needs to set the next pointer of the new node to the current head and update the head with the new node.

Where can it be used?

A linked list can be used in stacks, queues, and lists.

Code Example

final class ListNode {
    let val: Int
    var next: ListNode?

    init(val: Int, next: ListNode? = nil) {
        self.val = val
        self.next = next
    }
}

final class LinkedList {

    private(set) var head: ListNode?

    init(head: ListNode? = nil) {
        self.head = head
    }

    func addToTail(_ newNode: ListNode?) {
        if self.head == nil {
            self.head = newNode
            return
        }

        var node: ListNode? = self.head

        while node?.next != nil {
            node = node!.next
        }

        node?.next = newNode
    }

    func addToHead(_ newNode: ListNode?) {
        newNode?.next = self.head
        self.head = newNode
    }

}

Thank you for reading! 😊