Merge Two Sorted Lists

easy

Two warehouse conveyor lines each deliver packages already sorted by weight. You're building the merge station: splice both lines into a single line, still sorted, reusing the existing nodes — no new packages, just rerouted track.

Given the heads of two sorted linked lists, merge them into one sorted list and return its head. Tests pass plain arrays; implement the core in _merge on real nodes.

Example 1

in a = [1, 2, 4], b = [1, 3, 4]

out [1, 1, 2, 3, 4, 4]

Example 2

in a = [], b = []

out []

Example 3

in a = [], b = [0]

out [0]

Merging with an empty list returns the other list.

Constraints

· 0 <= nodes in each list <= 50
· -100 <= node value <= 100
· both inputs sorted ascending
· reuse nodes — no new allocations for data