Quick sort
Quicksort is a sorting algorithm based on the divide and conquer approach where an array is divided into subarrays by selecting a pivot element (element selected from the array).
- While dividing the array, the pivot element should be positioned in such a way that elements less than the pivot are kept on the left side, and elements greater than the pivot is on the right side of the pivot.
- The left and right subarrays are also divided using the same approach. This process continues until each subarray contains a single element.
- At this point, elements are already sorted. Finally, elements are combined to form a sorted array.
Working of Quick Sort algorithm:
To know the functioning of Quick sort, let’s consider an array arr[] = {10, 80, 30, 90, 40, 50, 70}
- Indexes: 0 1 2 3 4 5 6
- low = 0, high = 6, pivot = arr[h] = 70
- Initialize index of smaller element, i = -1
Step1
- Traverse elements from j = low to high-1
- j = 0: Since arr[j] <= pivot, do i++ and swap(arr[i], arr[j])
- i = 0
- arr[] = {10, 80, 30, 90, 40, 50, 70} // No change as i and j are same
- j = 1: Since arr[j] > pivot, do nothing
Step2
- j = 2 : Since arr[j] <= pivot, do i++ and swap(arr[i], arr[j])
- i = 1
- arr[] = {10, 30, 80, 90, 40, 50, 70} // We swap 80 and 30
Step3
- j = 3 : Since arr[j] > pivot, do nothing // No change in i and arr[]
- j = 4 : Since arr[j] <= pivot, do i++ and swap(arr[i], arr[j])
- i = 2
- arr[] = {10, 30, 40, 90, 80, 50, 70} // 80 and 40 Swapped
Step 4
- j = 5 : Since arr[j] <= pivot, do i++ and swap arr[i] with arr[j]
- i = 3
- arr[] = {10, 30, 40, 50, 80, 90, 70} // 90 and 50 Swapped
Step 5
- We come out of loop because j is now equal to high-1.
- Finally we place pivot at correct position by swapping arr[i+1] and arr[high] (or pivot)
- arr[] = {10, 30, 40, 50, 70, 90, 80} // 80 and 70 Swapped
Step 6
- Now 70 is at its correct place. All elements smaller than 70 are before it and all elements greater than 70 are after it.
- Since quick sort is a recursive function, we call the partition function again at left and right partitions
Step 7
- Again call function at right part and swap 80 and 90
Step 8
Introduction to Sorting Techniques – Data Structure and Algorithm Tutorials
Sorting refers to rearrangement of a given array or list of elements according to a comparison operator on the elements. The comparison operator is used to decide the new order of elements in the respective data structure.
When we have a large amount of data, it can be difficult to deal with it, especially when it is arranged randomly. When this happens, sorting that data becomes crucial. It is necessary to sort data in order to make searching easier.
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