 payal-kothari
6/22/2017 - 7:07 AM

## quicksort.java

https://www.youtube.com/watch?v=3DV8GO9g7B4

// partition step is always O(n) // recusrsive call time complexity changes depending on the given input best case and avg case = O(nlogn) worst case = O(n^2)

similar to "move zeros" leetcode problem

``````// 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<high; j++)
{
// If current element is smaller than or
// equal to pivot
if (arr[j] <= pivot)
{
i++;

// swap arr[i] and arr[j]
int temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
}
}

// swap arr[i+1] and arr[high] (or pivot)
int temp = arr[i+1];
arr[i+1] = arr[high];
arr[high] = temp;

return i+1;
}

/* The main function that implements QuickSort()
arr[] --> 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<n; ++i)
System.out.print(arr[i]+" ");
System.out.println();
}

// Driver program
public static void main(String args[])
{
int arr[] = {10, 7, 8, 9, 1, 5};
int n = arr.length;

QuickSort ob = new QuickSort();
ob.sort(arr, 0, n-1);

System.out.println("sorted array");
printArray(arr);
}
}
/*This code is contributed by Rajat Mishra */``````