In merging two sorted lists in place, what is the technique used?

• A. Create a new merged list by allocating new nodes.
• B. Reuse existing nodes by adjusting next pointers while merging; O(m+n) time, O(1) space.
• C. Sort both lists first, then merge.
• D. If one list ends, discard it.

Answer: (B)

In-place merging of two sorted linked lists relies on re-linking the existing nodes rather than creating new ones. You keep two pointers to the current nodes in each list and a tail pointer for the merged result. At every step, you compare the two current nodes and attach the smaller one to the tail, then advance that list and move the tail forward. This preserves the original nodes and uses only constant extra space, while touching each node once, giving O(m+n) time with O(1) extra space. If one list runs out of nodes, you simply connect the tail to the remainder of the other list rather than discarding anything. Creating new nodes would use more space, sorting both lists again is unnecessary since they’re already sorted, and discarding a remainder would lose data.