Quickselect Algorithm

Quickselect is a selection algorithm to find the k-th smallest element in an unordered list. It is related to the quick sort sorting algorithm.

Examples:

Input: arr[] = {7, 10, 4, 3, 20, 15}
           k = 3
Output: 7

Input: arr[] = {7, 10, 4, 3, 20, 15}
           k = 4
Output: 10

code link

#include <stdio.h>

#define SWAP(x, y) { int temp = x; x = y; y = temp; }
#define N (sizeof(A)/sizeof(A[0]))

// Partition using Lomuto partition scheme
int partition(int a[], int left, int right, int pivotIndex)
{
// Pick pivotIndex as pivot from the array
int pivot = a[pivotIndex];

// Move pivot to end
SWAP(a[pivotIndex], a[right]);

// elements less than pivot will be pushed to the left of pIndex
// elements more than pivot will be pushed to the right of pIndex
// equal elements can go either way
int pIndex = left;
int i;

// each time we finds an element less than or equal to pivot, pIndex
// is incremented and that element would be placed before the pivot. 
for (i = left; i < right; i++)
{
if (a[i] <= pivot)
{
SWAP(a[i], a[pIndex]);
pIndex++;
}
}

// Move pivot to its final place
SWAP (a[pIndex], a[right]);

// return pIndex (index of pivot element)
return pIndex;
}

// Returns the k-th smallest element of list within left..right inclusive
// (i.e. left <= k <= right).
// The search space within the array is changing for each round - but the list
// is still the same size. Thus, k does not need to be updated with each round.

int quickselect(int A[], int left, int right, int k)
{
// If the array contains only one element, return that element
if (left == right)
return A[left];

// select a pivotIndex between left and right
int pivotIndex = left + rand() % (right - left + 1);

pivotIndex = partition(A, left, right, pivotIndex);

// The pivot is in its final sorted position
if (k == pivotIndex)
return A[k];

// if k is less than the pivot index
else if (k < pivotIndex)
return quickselect(A, left, pivotIndex - 1, k);

// if k is more than the pivot index
else
return quickselect(A, pivotIndex + 1, right, k);
}

// main function
int main()
{
int A[] = { 7, 4, 6, 3, 9, 1 };
int k = 2;

printf("K'th smallest element is %d", quickselect(A, 0, N - 1, k));

return 0;
}


c++ function:-

#include <iostream>
#include <vector>
#include <algorithm>

// main function
int main()
{
std::vector<int> a = { 7, 4, 6, 3, 9, 1 };
const std::size_t k = 2;

std::nth_element(a.begin(), a.begin() + k, a.end());
std::cout << "K'th smallest element is " << a[k];

return 0;
}