Sorting algorithms: Students should know the mechanics of the algorithm.

The idea

Knowing the mechanics of merge sort means being able to describe and follow each step precisely. The algorithm first divides: the list is split into two halves, and each half is split again, repeating until every sublist contains one item. Then it merges: two sorted sublists are combined by comparing their front items, copying the smaller into the next position of the output, and advancing that sublist. When one sublist runs out, the remaining items of the other are appended in order. Each merge creates a longer sorted sublist, and merging continues until one sorted list remains. For [5, 2, 9, 1], the splits give single items, then merges produce [2, 5] and [1, 9], and the final merge produces [1, 2, 5, 9].

What earns the marks

• Describes the divide stage: splitting the list into halves repeatedly until single-item sublists remain.
• Describes the merge stage: comparing the front items of two sorted sublists and placing the smaller next.
• Explains that when one sublist is exhausted, the remaining items of the other sublist are appended.
• States that each merge produces a larger sorted sublist and merging continues until one sorted list remains.
• Traces the mechanics correctly on a short example, showing both the splits and the ordered merges.
• Distinguishes the order of operations: all splitting happens before merging begins.

Common mistakes and how to put them right

• Confusing merge sort with an in-place swapping algorithm.

  Put it right: Merge sort divides into sublists and merges them into an ordered sequence.

• Merging sublists without comparing items.

  Put it right: The front items must be compared so the smaller is placed next.

• Forgetting to append leftover items when one sublist is exhausted.

  Put it right: The remaining items are copied in order to complete the merge.

• Mixing up the stages by merging before splitting is complete.

  Put it right: The list is fully divided into single items before merging starts.

Exam tips

• Write out the split tree for a short list, then perform each merge step by step to check your understanding of the mechanics.
• Use precise vocabulary: divide, sublist, compare, merge, append, sorted.
• When tracing, keep the two sublists side by side and mark which item is copied at each comparison.
• Verify the final list is fully ordered and contains every original item exactly once.