Given two sorted linked lists list1 and list2, merge them into a single sorted linked list by splicing the existing nodes together — no new nodes should be created. The recursive framing turns the merge into a chain of small decisions: compare the two current heads, let the smaller one lead, and set its `next` pointer to the result of recursively merging everything after it with the other list untouched. The base cases are immediate — merging anything with an empty list is just that list, unchanged. Every call makes exactly one decision and hands off a strictly smaller version of the same problem, so the total number of calls (and the total work) is proportional to the combined length of both lists — the same O(n+m) as the iterative two-pointer approach, just paying for it in call-stack frames instead of a fixed pair of pointers.