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Consider the problem of reversing a singly linked list. To take an example, given the linked list below, head -> a -> b -> c -> d -> e -> NULL, the reversed linked list should look like head -> e -> d -> c -> b -> a -> NULL. Which one of the following statements is TRUE about the time complexity of algorithms that solve the above problem in O(1) space?

1. The best algorithm for the problem takes θ(n) time in the worst case.
2. The best algorithm for the problem takes θ(n log n) time in the worst case.
3. The best algorithm for the problem takes θ(n²) time in the worst case.
4. It is not possible to reverse a singly linked list in O(1) space.

Correct answer: The best algorithm for the problem takes θ(n) time in the worst case.

Solution

Reversing a singly linked list requires visiting each node exactly once to change the pointers, which results in a linear time complexity of θ(n). This is efficient and feasible within O(1) space, as it only requires a few additional pointers for the reversal process.

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