我们建议阅读以下文章，作为该文章的先决条件。
未排序数组中第K个最小/最大元素|套装1
给定一个数组和一个数字k，其中k小于数组的大小，我们需要找到给定数组中第k个最小的元素。假定所有数组元素都是不同的。
例子：
Input: arr[] = {7, 10, 4, 3, 20, 15}
k = 3
Output: 7
Input: arr[] = {7, 10, 4, 3, 20, 15}
k = 4
Output: 10我们在这里讨论了三种不同的解决方案。
在这篇文章中，讨论了方法5，它主要是前一篇文章中讨论的方法4(QuickSelect)的扩展。这个想法是随机选择一个枢轴元素。为了实现随机分区，我们使用随机函数rand()在l和r之间生成索引，将随机生成的索引处的元素与最后一个元素交换，最后调用将最后一个元素用作枢轴的标准分区过程。
以下是上述随机快速选择的实现。
输出：
K'th smallest element is 5C++
// C++ implementation of randomized quickSelect
#include
#include
#include
using namespace std;
int randomPartition(int arr[], int l, int r);
// This function returns k'th smallest element in arr[l..r] using
// QuickSort based method. ASSUMPTION: ELEMENTS IN ARR[] ARE DISTINCT
int kthSmallest(int arr[], int l, int r, int k)
{
// If k is smaller than number of elements in array
if (k > 0 && k <= r - l + 1)
{
// Partition the array around a random element and
// get position of pivot element in sorted array
int pos = randomPartition(arr, l, r);
// If position is same as k
if (pos-l == k-1)
return arr[pos];
if (pos-l > k-1) // If position is more, recur for left subarray
return kthSmallest(arr, l, pos-1, k);
// Else recur for right subarray
return kthSmallest(arr, pos+1, r, k-pos+l-1);
}
// If k is more than the number of elements in the array
return INT_MAX;
}
void swap(int *a, int *b)
{
int temp = *a;
*a = *b;
*b = temp;
}
// Standard partition process of QuickSort(). It considers the last
// element as pivot and moves all smaller element to left of it and
// greater elements to right. This function is used by randomPartition()
int partition(int arr[], int l, int r)
{
int x = arr[r], i = l;
for (int j = l; j <= r - 1; j++)
{
if (arr[j] <= x)
{
swap(&arr[i], &arr[j]);
i++;
}
}
swap(&arr[i], &arr[r]);
return i;
}
// Picks a random pivot element between l and r and partitions
// arr[l..r] around the randomly picked element using partition()
int randomPartition(int arr[], int l, int r)
{
int n = r-l+1;
int pivot = rand() % n;
swap(&arr[l + pivot], &arr[r]);
return partition(arr, l, r);
}
// Driver program to test above methods
int main()
{
int arr[] = {12, 3, 5, 7, 4, 19, 26};
int n = sizeof(arr)/sizeof(arr[0]), k = 3;
cout << "K'th smallest element is " << kthSmallest(arr, 0, n-1, k);
return 0;
} Java
// Java program to find k'th smallest element in expected
// linear time
class KthSmallst
{
// This function returns k'th smallest element in arr[l..r]
// using QuickSort based method. ASSUMPTION: ALL ELEMENTS
// IN ARR[] ARE DISTINCT
int kthSmallest(int arr[], int l, int r, int k)
{
// If k is smaller than number of elements in array
if (k > 0 && k <= r - l + 1)
{
// Partition the array around a random element and
// get position of pivot element in sorted array
int pos = randomPartition(arr, l, r);
// If position is same as k
if (pos-l == k-1)
return arr[pos];
// If position is more, recur for left subarray
if (pos-l > k-1)
return kthSmallest(arr, l, pos-1, k);
// Else recur for right subarray
return kthSmallest(arr, pos+1, r, k-pos+l-1);
}
// If k is more than number of elements in array
return Integer.MAX_VALUE;
}
// Utility method to swap arr[i] and arr[j]
void swap(int arr[], int i, int j)
{
int temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
}
// Standard partition process of QuickSort(). It considers
// the last element as pivot and moves all smaller element
// to left of it and greater elements to right. This function
// is used by randomPartition()
int partition(int arr[], int l, int r)
{
int x = arr[r], i = l;
for (int j = l; j <= r - 1; j++)
{
if (arr[j] <= x)
{
swap(arr, i, j);
i++;
}
}
swap(arr, i, r);
return i;
}
// Picks a random pivot element between l and r and
// partitions arr[l..r] arount the randomly picked
// element using partition()
int randomPartition(int arr[], int l, int r)
{
int n = r-l+1;
int pivot = (int)(Math.random()) * (n-1);
swap(arr, l + pivot, r);
return partition(arr, l, r);
}
// Driver method to test above
public static void main(String args[])
{
KthSmallst ob = new KthSmallst();
int arr[] = {12, 3, 5, 7, 4, 19, 26};
int n = arr.length,k = 3;
System.out.println("K'th smallest element is "+
ob.kthSmallest(arr, 0, n-1, k));
}
}
/*This code is contributed by Rajat Mishra*/Python3
# Python3 implementation of randomized
# quickSelect
import random
# This function returns k'th smallest
# element in arr[l..r] using QuickSort
# based method. ASSUMPTION: ELEMENTS
# IN ARR[] ARE DISTINCT
def kthSmallest(arr, l, r, k):
# If k is smaller than number of
# elements in array
if (k > 0 and k <= r - l + 1):
# Partition the array around a random
# element and get position of pivot
# element in sorted array
pos = randomPartition(arr, l, r)
# If position is same as k
if (pos - l == k - 1):
return arr[pos]
if (pos - l > k - 1): # If position is more,
# recur for left subarray
return kthSmallest(arr, l, pos - 1, k)
# Else recur for right subarray
return kthSmallest(arr, pos + 1, r,
k - pos + l - 1)
# If k is more than the number of
# elements in the array
return 999999999999
def swap(arr, a, b):
temp = arr[a]
arr[a] = arr[b]
arr[b] = temp
# Standard partition process of QuickSort().
# It considers the last element as pivot and
# moves all smaller element to left of it and
# greater elements to right. This function
# is used by randomPartition()
def partition(arr, l, r):
x = arr[r]
i = l
for j in range(l, r):
if (arr[j] <= x):
swap(arr, i, j)
i += 1
swap(arr, i, r)
return i
# Picks a random pivot element between l and r
# and partitions arr[l..r] around the randomly
# picked element using partition()
def randomPartition(arr, l, r):
n = r - l + 1
pivot = int(random.random() * n)
swap(arr, l + pivot, r)
return partition(arr, l, r)
# Driver Code
if __name__ == '__main__':
arr = [12, 3, 5, 7, 4, 19, 26]
n = len(arr)
k = 3
print("K'th smallest element is",
kthSmallest(arr, 0, n - 1, k))
# This code is contributed by PranchalKC#
// C# program to find k'th smallest
// element in expected linear time
using System;
class GFG
{
// This function returns k'th smallest
// element in arr[l..r] using QuickSort
// based method. ASSUMPTION: ALL ELEMENTS
// IN ARR[] ARE DISTINCT
int kthSmallest(int []arr, int l, int r, int k)
{
// If k is smaller than number
// of elements in array
if (k > 0 && k <= r - l + 1)
{
// Partition the array around a
// random element and get position
// of pivot element in sorted array
int pos = randomPartition(arr, l, r);
// If position is same as k
if (pos-l == k - 1)
return arr[pos];
// If position is more, recur
// for left subarray
if (pos - l > k - 1)
return kthSmallest(arr, l, pos - 1, k);
// Else recur for right subarray
return kthSmallest(arr, pos + 1, r,
k - pos + l - 1);
}
// If k is more than number of
// elements in array
return int.MaxValue;
}
// Utility method to swap arr[i] and arr[j]
void swap(int []arr, int i, int j)
{
int temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
}
// Standard partition process of QuickSort().
// It considers the last element as pivot and
// moves all smaller element to left of it
// and greater elements to right. This function
// is used by randomPartition()
int partition(int []arr, int l, int r)
{
int x = arr[r], i = l;
for (int j = l; j <= r - 1; j++)
{
if (arr[j] <= x)
{
swap(arr, i, j);
i++;
}
}
swap(arr, i, r);
return i;
}
// Picks a random pivot element between
// l and r and partitions arr[l..r]
// around the randomly picked element
// using partition()
int randomPartition(int []arr, int l, int r)
{
int n = r - l + 1;
Random rnd = new Random();
int rand = rnd.Next(0, 1);
int pivot = (int)(rand * (n - 1));
swap(arr, l + pivot, r);
return partition(arr, l, r);
}
// Driver Code
public static void Main()
{
GFG ob = new GFG();
int []arr = {12, 3, 5, 7, 4, 19, 26};
int n = arr.Length,k = 3;
Console.Write("K'th smallest element is "+
ob.kthSmallest(arr, 0, n - 1, k));
}
}
// This code is contributed by 29AjayKumarPHP
0 && $k <= $r - $l + 1)
{
// Partition the array around a random element and
// get position of pivot element in sorted array
$pos = randomPartition($arr, $l, $r);
// If position is same as k
if ($pos-$l == $k-1)
return $arr[$pos];
// If position is more, recur for left subarray
if ($pos-$l > $k-1)
return kthSmallest($arr, $l, $pos-1, $k);
// Else recur for right subarray
return kthSmallest($arr, $pos+1, $r,
$k-$pos+$l-1);
}
// If k is more than the number of elements in the array
return PHP_INT_MAX;
}
function swap($a, $b)
{
$temp = $a;
$a = $b;
$b = $temp;
}
// Standard partition process of QuickSort().
// It considers the last element as pivot
// and moves all smaller element to left
// of it and greater elements to right.
// This function is used by randomPartition()
function partition($arr, $l, $r)
{
$x = $arr[$r];
$i = $l;
for ($j = $l; $j <= $r - 1; $j++)
{
if ($arr[$j] <= $x)
{
list($arr[$i], $arr[$j])=array($arr[$j],$arr[$i]);
//swap(&arr[i], &arr[j]);
$i++;
}
}
list($arr[$i], $arr[$r])=array($arr[$r],$arr[$i]);
//swap(&arr[i], &arr[r]);
return $i;
}
// Picks a random pivot element between
// l and r and partitions arr[l..r] around
// the randomly picked element using partition()
function randomPartition($arr, $l, $r)
{
$n = $r-$l+1;
$pivot = rand() % $n;
list($arr[$l + $pivot], $arr[$r]) =
array($arr[$r],$arr[$l + $pivot] );
//swap(&arr[l + pivot], &arr[r]);
return partition($arr, $l, $r);
}
// Driver program to test the above methods
$arr = array(12, 3, 5, 7, 4, 19, 260);
$n = sizeof($arr)/sizeof($arr[0]);
$k = 3;
echo "K'th smallest element is " ,
kthSmallest($arr, 0, $n-1, $k);
// This code is contributed by ajit.
?>时间复杂度：
上述解决方案的最坏情况下的时间复杂度仍然是O(n 2 )。在最坏的情况下，随机函数可能总是选择一个角元素。以上随机化QuickSelect的预期时间复杂度为O(n)，有关证明，请参阅CLRS书或MIT视频讲座。分析中的假设是，随机数生成器同样有可能生成输入范围内的任何数字。
资料来源：
麻省理工学院有关订单统计的视频讲座，中位数
Clifford Stein，Thomas H.Cormen，Charles E.Leiserson，Ronald L.