Insert at Head - Python DSA

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

Algorithm

Basic Implementation

basic.py

class Node:
    __slots__ = ("value", "next")
    def __init__(self, value, nxt=None):
        self.value = value
        self.next = nxt

def render(head):
    parts = []
    cursor = head
    while cursor is not None:
        parts.append(str(cursor.value))
        cursor = cursor.next
    return " -> ".join(parts) + " -> null"

head = Node(20, Node(30))
new_head = Node(10)
new_head.next = head
head = new_head
print(render(head))

Head insertion changes only two references: the new node points at the old head, then head moves to the new node.

Step 1 - Old first node

Before insertion, head points at node(20).

Step 2 - New node links to old head

Set new.next to the old first node before moving head.

Step 3 - Head moves to the new node

The final chain has 10 first: 10 -> 20 -> 30 -> null.

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.