Insertion Sort is a simple sorting algorithm that builds the final sorted array (or list) one item at a time. It is much less efficient on large lists than more advanced algorithms such as quicksort, heapsort, or merge sort.
Given below is an unsorted array. Insertion sort takes O(n) time in Best Case and Ο(n2) time for Average and Worst Case.
Insertion sort compares the first two elements
It finds that both 14 and 33 are already in ascending order. For now, 14 is in sorted sub-list.
Insertion sort moves ahead and compares 33 with 27.
And finds that 33 is not in the correct position.
It swaps 33 with 27. It also checks with all the elements of sorted sub-list. Here we see that the sorted sub-list has only one element 14, and 27 is greater than 14. Hence, the sorted sub-list remains sorted after swapping.
By now we have 14 and 27 in the sorted sub-list. Next, it compares 33 with 10.
These values are not in a sorted order.
So we swap them.
However, swapping makes 27 and 10 unsorted.
Hence, we swap them too.
Again we find 14 and 10 in an unsorted order.
We swap them again. By the end of third iteration, we have a sorted sub-list of 4 items.
This process goes on until all the unsorted values are covered in a sorted sub-list. Now we shall see some programming aspects of insertion sort.
Step 1 − If it is the first element, it is already sorted. return 1;
Step 2 − Pick next element
Step 3 − Compare with all elements in the sorted sub-list
Step 4 − Shift all the elements in the sorted sub-list that is greater than the value to be sorted
Step 5 − Insert the value
Step 6 − Repeat until list is sorted
Pseudocode of InsertionSort algorithm can be expressed as −
procedure insertionSort( A : array of items )
int holePosition
int valueToInsert
for i = 1 to length(A) inclusive do:
/* select value to be inserted */
valueToInsert = A[i]
holePosition = i
/*locate hole position for the element to be inserted */
while holePosition > 0 and A[holePosition-1] > valueToInsert do:
A[holePosition] = A[holePosition-1]
holePosition = holePosition -1
end while
/* insert the number at hole position */
A[holePosition] = valueToInsert
end for
end procedure
Time complexity
Best Case: O(n)
Average and Worst Case: О(n2)
where n is the number of items being sorted.
Space complexity - O(1), due to auxillary space only.