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Insertion Sort - Python - GeeksforGeeks

Last Updated : 23 Jul, 2025

Insertion sort is a simple sorting algorithm that works by iteratively inserting each element of an unsorted list into its correct position in a sorted portion of the list.

• The insertionSort function takes an array arr as input. It first calculates the length of the array (n). If the length is 0 or 1, the function returns immediately as an array with 0 or 1 element is considered already sorted.
• For arrays with more than one element, We start with second element of the array as first element in the array is assumed to be sorted.
• Compare second element with the first element and check if the second element is smaller then swap them.
• Move to the third element and compare it with the first two elements and put at its correct position
• Repeat until the entire array is sorted.

The provided example demonstrates the sorting process using the insertion sort algorithm. The initial array [12, 11, 13, 5, 6] is subjected to the insertionSort function. After sorting, the array should be [5, 6, 11, 12, 13]. The code prints the sorted array as the final output.

def insertionSort(arr):
    n = len(arr)  # Get the length of the array
     
    if n <= 1:
        return  # If the array has 0 or 1 element, it is already sorted, so return

    for i in range(1, n):  # Iterate over the array starting from the second element
        key = arr[i]  # Store the current element as the key to be inserted in the right position
        j = i-1
        while j >= 0 and key < arr[j]:  # Move elements greater than key one position ahead
            arr[j+1] = arr[j]  # Shift elements to the right
            j -= 1
        arr[j+1] = key  # Insert the key in the correct position
 
# Sorting the array [12, 11, 13, 5, 6] using insertionSort
arr = [12, 11, 13, 5, 6]
insertionSort(arr)
print(arr)

Output:

Sorted array is:
[5, 6, 11, 12, 13]

Time Complexity: O(N²) 
Auxiliary Space: O(1)

Please refer complete article on Insertion Sort for more details!

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