Is Insertion Sort Stable
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Is Insertion Sort Stable?

Insertion sort is an efficient sorting algorithm that is commonly used to sort small data sets or partially sortedlists. However, one question that often arises is whether insertion sort is a stable sorting algorithm. In thisarticle, we will explore the stability of insertion sort and provide a comprehensive explanation.

Understanding Stability in Sorting Algorithms

Before diving into the stability of insertion sort, it is essential to understand what stability means in thecontext of sorting algorithms. A stable sorting algorithm maintains the relative order of elements with equalkeys. In other words, if there are two elements with the same key, the stable algorithm ensures that theiroriginal order is preserved in the sorted output.

How Insertion Sort Works

Insertion sort works by dividing the input into two parts: the sorted portion and the unsorted portion. Initially,the sorted portion contains only the first element, and the unsorted portion contains the rest of the elements.The algorithm iterates through the unsorted portion, comparing each element with the elements in the sortedportion, and inserts it at the correct position to maintain the sorted order.

Proof of Stability in Insertion Sort

Now, let's analyze why insertion sort is a stable sorting algorithm:

Case 1: Elements with Unique Keys

When all the elements in the input have unique keys, the relative order of these elements remains the samethroughout the sorting process. Since insertion sort compares elements one by one and only moves an element ifit is greater than the key, the original order is always preserved. Hence, insertion sort is stable in thisscenario.

Case 2: Elements with Duplicate Keys

Insertion sort also maintains stability when there are elements with duplicate keys. Consider the followingexample:

Input: [5A, 3, 5B, 2, 1]

In this case, there are two elements with the key 5. During the sorting process, when the second element with thekey 5 is compared with the first element with the same key, insertion sort does not move it to the left if theyare equal. This behavior ensures that the relative order of elements with equal keys is preserved. Therefore,insertion sort is stable even when there are duplicate keys.

Advantages and Disadvantages of Insertion Sort


  • Efficient for small data sets or partially sorted lists.
  • Simple implementation and easy to understand.
  • In-place sorting algorithm, meaning it does not require additional memory.
  • Stable sorting algorithm, as explained above.


  • Not suitable for large data sets due to its time complexity of O(n^2).
  • Less efficient compared to other advanced sorting algorithms like merge sort or quicksort.

Frequently Asked Questions (FAQs)

Q: Can insertion sort handle large data sets efficiently?

A: Insertion sort is not recommended for large data sets because its time complexity of O(n^2) makes it slowercompared to more advanced sorting algorithms.

Q: Is insertion sort always stable?

A: Yes, insertion sort is a stable sorting algorithm as it maintains the relative order of elements with equalkeys.

Q: Is insertion sort considered an efficient sorting algorithm?

A: Insertion sort is efficient for small data sets or partially sorted lists. However, for large data sets, othersorting algorithms like merge sort or quicksort are more suitable.


In conclusion, insertion sort is indeed a stable sorting algorithm. It preserves the relative order of elementswith equal keys, making it reliable for scenarios where stability is a requirement. However, it is important tonote that insertion sort's efficiency decreases significantly for large data sets. Therefore, it is advisable toconsider other sorting algorithms for such cases. Nevertheless, for small data sets or partially sorted lists,insertion sort provides a simple and effective solution.

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