In sorting linked lists, what does stability mean, and is merge sort stable for linked lists?

• A. Stability means faster performance; merge sort is not stable
• B. Stability preserves the relative order of equal elements; merge sort on linked lists is stable
• C. Stability becomes worse as the list grows
• D. Stability relates to memory usage rather than order

Answer: (B)

Stability in sorting means preserving the relative order of elements that compare as equal. When you sort a linked list, this matters because you might have tied keys from an earlier pass or from another attribute; a stable sort keeps those equal items in the same order they appeared in the input.

Merge sort on linked lists is stable because the merge step can be done so that, when two nodes have the same key, you take the node from the left list first. This preserves the original ordering of equal elements as you combine the two sorted halves. Since the overall process relies on these stable merge operations, the entire sort remains stable.

Other statements miss the essence of stability. Stability is about ordering of equal elements, not about speed, memory usage, or how the size of the list affects stability.