Linked Structures

Insert at Head

Insert a new first node by pointing it at the old head and then moving the head pointer.

Algorithm

The replay labels nodes by value, such as node(20), and never exposes object identity or memory addresses. This Swift DSA implementation uses the same small chain as the rest of the DSA track.

Basic Implementation

basic.swift 
final class Node {
    let value: Int
    var next: Node?
    init(_ value: Int, _ next: Node? = nil) {
        self.value = value
        self.next = next
    }
}

func render(_ head: Node?) -> String {
    var parts: [String] = []
    var cursor = head
    while cursor != nil {
        parts.append(String(cursor!.value))
        cursor = cursor!.next
    }
    return parts.joined(separator: " -> ") + " -> null"
}

var head: Node? = Node(20, Node(30))
let newHead = Node(10)
newHead.next = head
head = newHead
print(render(head))

Complexity

  • Time: O(1)
  • Space: O(1)

Implementation notes

  • Keep the explicit node and pointer/reference operations; array shortcuts hide the linked-list state this lesson is meant to replay.
  • The final output prints the chain in a deterministic a -> b -> null form for cross-language comparison.
old head The previous first node becomes the second node.
constant-time insert Only the new node and head pointer change.