Sorting Stability and When to Use Which Sort in DSA C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean for a sorting algorithm to be stable?

A stable sorting algorithm keeps the original order of equal elements the same after sorting.

beginner

Name a common stable sorting algorithm.

Merge Sort is a common stable sorting algorithm.

intermediate

Why might you choose a stable sort over an unstable sort?

You choose a stable sort when you want to keep the relative order of equal elements, for example, when sorting by multiple keys step-by-step.

intermediate

Which sorting algorithm is generally fastest for small arrays and why?

Insertion Sort is generally fastest for small arrays because it has low overhead and works well when the array is nearly sorted.

intermediate

When should you prefer Quick Sort over Merge Sort?

Prefer Quick Sort when you want an in-place sort with good average performance and less memory use, but be aware it is not stable.

Which of these sorting algorithms is stable?

A Merge Sort

B Quick Sort

C Heap Sort

D Selection Sort

Merge Sort is stable because it preserves the order of equal elements during merging.

Why is Insertion Sort preferred for small or nearly sorted arrays?

A It uses extra memory

B It has low overhead and adapts well to nearly sorted data

C It is unstable

D It always runs in O(n log n)

Insertion Sort is simple and efficient for small or nearly sorted arrays because it minimizes comparisons and swaps.

Which sorting algorithm is NOT stable?

A Bubble Sort

B Merge Sort

C Quick Sort

D Insertion Sort

Quick Sort is generally not stable because it swaps elements without preserving order of equal keys.

When sorting large data that does not fit in memory, which algorithm is preferred?

A Quick Sort

B Insertion Sort

C Selection Sort

D Merge Sort

Merge Sort is preferred for large data and external sorting because it can be implemented to work with data on disk.

Which sorting algorithm uses divide and conquer and is stable?

A Merge Sort

B Heap Sort

C Quick Sort

D Selection Sort

Merge Sort uses divide and conquer and is stable.

Explain what sorting stability means and give an example of when it matters.

Think about sorting a list of people first by age, then by name.

Describe when you would choose Quick Sort, Merge Sort, or Insertion Sort.

Consider memory use, stability, and data size.