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Insertion at the head of Linked List

Easy

Point the new node at the old head

Problem

Given the head of a singly linked list and a value, insert a new node with that value at the front so it becomes the new head. Return the new head.

In simple words

Make a new node point at the old first node, then call it the new first.

The idea

Create the node, set its next to the current head, and return it as the new head. Order matters: link the new node first, because reassigning the head before doing so loses the rest of the list.

The trick

  • newNode.next = head, then head = newNode. Never the other way round.
  • O(1), and it works on an empty list without a special case.

This one walks through the worked example rather than tracing the algorithm frame by frame — a full walkthrough is still to be drawn. The code and the idea below are the real solution.

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Step 1 of 2. Here's the example — head = [2, 3, 4], val = 1 Values: 2, 3, 4.

1/2
Optimal
timeO(1)spaceO(1)
1newNode = Node(val)2newNode.next = head3head = newNode4return head

Input

array
[2, 3, 4]

Output

answer
—

Check yourself

3 quick questions about this walkthrough. A wrong answer costs nothing.

Example

Input:
head = [2, 3, 4], val = 1
Output:
[1, 2, 3, 4]

Practice this problem:GeeksforGeeks(opens in a new tab)

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