What is the time complexity of insertFirst() in an unordered linked list?

• A. O(1)
• B. O(n)
• C. O(log n)
• D. O(n log n)

Answer: (A)

In a linked list, inserting at the front is constant-time because you only do a few pointer updates: create a new node, point its next to the current head, and update the head to this new node. This work does not depend on how many nodes are already in the list, so the time complexity is O(1). The unordered nature doesn’t change this; it just means there’s no reliance on any specific order, but front insertion still requires no traversal. If you were inserting at the end without a tail pointer, that could be O(n), but for front insertion the cost remains constant.