AI Mentor for Computer Science Students

O(n log n) — every level of the recursion does O(n) work merging, and there are log n levels since the array halves each time. Why it matters for viva: examiners often follow up with "why not O(n)?" — say it plainly: you can't sort by looking at each element once, you need comparisons across the whole array, and log n splits is the minimum needed to get there.