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# Java Program for QuickSort

• Difficulty Level : Medium
• Last Updated : 13 Jun, 2022

Like Merge Sort, QuickSort is a Divide and Conquer algorithm. It picks an element as pivot and partitions the given array around the picked pivot. There are many different versions of quickSort that pick pivot in different ways.

1. Always pick first element as pivot.
2. Always pick last element as pivot (implemented below)
3. Pick a random element as pivot.
4. Pick median as pivot.

The key process in quickSort is partition(). Target of partitions is, given an array and an element x of array as pivot, put x at its correct position in sorted array and put all smaller elements (smaller than x) before x, and put all greater elements (greater than x) after x. All this should be done in linear time. Pseudo Code for recursive QuickSort function :

```/* low  --> Starting index,  high  --> Ending index */
quickSort(arr[], low, high)
{
if (low < high)
{
/* pi is partitioning index, arr[p] is now
at right place */
pi = partition(arr, low, high);

quickSort(arr, low, pi - 1);  // Before pi
quickSort(arr, pi + 1, high); // After pi
}
}```

## Java

 `// Java program for implementation of QuickSort` `class` `QuickSort` `{` `    ``/* This function takes last element as pivot,` `       ``places the pivot element at its correct` `       ``position in sorted array, and places all` `       ``smaller (smaller than pivot) to left of` `       ``pivot and all greater elements to right` `       ``of pivot */` `    ``int` `partition(``int` `arr[], ``int` `low, ``int` `high)` `    ``{` `        ``int` `pivot = arr[high]; ` `        ``int` `i = (low-``1``); ``// index of smaller element` `        ``for` `(``int` `j=low; j Array to be sorted,` `      ``low  --> Starting index,` `      ``high  --> Ending index */` `    ``void` `sort(``int` `arr[], ``int` `low, ``int` `high)` `    ``{` `        ``if` `(low < high)` `        ``{` `            ``/* pi is partitioning index, arr[pi] is ` `              ``now at right place */` `            ``int` `pi = partition(arr, low, high);`   `            ``// Recursively sort elements before` `            ``// partition and after partition` `            ``sort(arr, low, pi-``1``);` `            ``sort(arr, pi+``1``, high);` `        ``}` `    ``}`   `    ``/* A utility function to print array of size n */` `    ``static` `void` `printArray(``int` `arr[])` `    ``{` `        ``int` `n = arr.length;` `        ``for` `(``int` `i=``0``; i

Time Complexity: Worst case time complexity is O(N2) and average case time complexity is O(N logN)

Auxiliary Space: O(1)

Please refer complete article on QuickSort for more details!

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