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 Java DSA implementation uses the same small chain as the rest of the DSA track.

Basic Implementation

Basic.java 
public class Basic {
    static class Node {
        int value;
        Node next;
        Node(int value) { this.value = value; }
        Node(int value, Node next) { this.value = value; this.next = next; }
    }
    static String render(Node head) {
        StringBuilder out = new StringBuilder();
        Node cursor = head;
        while (cursor != null) {
            if (out.length() > 0) out.append(" -> ");
            out.append(cursor.value);
            cursor = cursor.next;
        }
        return out.append(" -> null").toString();
    }
    public static void main(String[] args) {
        Node head = new Node(20, new Node(30));
        Node newHead = new Node(10);
        newHead.next = head;
        head = newHead;
        System.out.println(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.