What is the time complexity of insertBack() in a doubly linked list?

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

Answer: (B)

Inserting at the back of a doubly linked list is performed in constant time when the list maintains a direct reference to its tail. You create the new node, link its prev to the current tail, set the current tail’s next to the new node, and then update the tail reference to this new node, all in a fixed number of steps. Since the number of operations doesn’t grow with the list size, the time complexity is O(1). This is why insertBack is constant time in a typical implementation with a tail pointer. If you didn’t have a tail pointer and had to walk from the head to find the end, it would be O(n), but with a tail reference it remains constant.