Stability in sorting means whether a sort algorithm maintains the relative order of the equals keys of the original input in the result output.

So a sorting algorithm is said to be stable if two objects with equal keys appear in the same order in sorted output as they appear in the input unsorted array.

Consider a list of pairs:

(1, 2) (9, 7) (3, 4) (8, 6) (9, 3)

Now we will sort the list using the first element of each pair.

A stable sorting of this list will output the below list:

(1, 2) (3, 4) (8, 6) (9, 7) (9, 3)

Because (9, 3) appears after (9, 7) in the original list as well.

An unstable sorting will output the below list:

(1, 2) (3, 4) (8, 6) (9, 3) (9, 7)

Unstable sort may generate the same output as the stable sort but not always.

Well-known stable sorts:

• Merge sort (opens new window)
• Insertion sort (opens new window)
• Radix sort (opens new window)
• Tim sort
• Bubble Sort (opens new window)

Well-known unstable sorts:

• Heap sort (opens new window)
• Quick sort (opens new window)

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