Combine Two Sorted Linked Lists

You're given the heads of two singly linked lists, each already sorted in ascending order. Implement linkedListCombineTwoSorted(a, b) so it weaves them into a single ascending-sorted list and returns its head. Each node is a plain object of the shape { val, next }, where next points at the following node or is null at the end of the list. This is the classic merge step that sits at the heart of merge sort — given two sorted runs, produce one.

Signature

// A node is a plain object: { val: number, next: Node | null }.
// a, b: Node | null   — the heads of two ascending-sorted lists (null = empty).
// returns: Node | null — the head of one ascending-sorted list (null if both empty).
function linkedListCombineTwoSorted(a, b): Node | null;

Examples

// a: 1 -> 3 -> 5,  b: 2 -> 4 -> 6  (shown as arrays for brevity)
linkedListCombineTwoSorted(list([1, 3, 5]), list([2, 4, 6]));
// → 1 -> 2 -> 3 -> 4 -> 5 -> 6

// Duplicate values across both lists are all kept.
linkedListCombineTwoSorted(list([1, 1, 2]), list([1, 3]));
// → 1 -> 1 -> 1 -> 2 -> 3

Notes

• Node shape. Every node is { val, next }. The list ends when a node's next is null. There is no separate ListNode class — just these plain objects.
• Inputs are already sorted. Both a and b arrive in ascending order. Lean on that — you do not need to sort anything from scratch.
• Splice, don't rebuild. Re-link the existing nodes into the merged order rather than allocating a fresh node per value. The only new node you should need is a single dummy head.
• Empty inputs. Either head may be null. If one is empty, return the other; if both are empty, return null.
• Don't mutate values. You may rewrite next pointers to splice nodes together, but leave each node's val alone.