线性时间复杂度求数组第n大

算法理论基础见《算法导论》的第9章

#include <iostream>
#include <time.h>
using namespace std;

#define NSIZ 1000

int partision(int arr[], int left, int right)
{
	int tmp = arr[left];


	while(left < right)
	{
		while(left < right && tmp <= arr[right]) --right;
		arr[left] = arr[right];


		while(left < right && arr[left] <= tmp) ++left;
		arr[right] = arr[left];
	}


	arr[left] = tmp;
	return left;


}

int random(int m, int n)
{
	if(m == n)
	{
		return m;
	}


	srand(time(0));


	return m + ((n - m) * rand()) / RAND_MAX;


}

int RandomPartision(int arr[], int left, int right)
{
	int t = random(left, right);
	int tmp = arr[t];
	arr[t] = arr[left];
	arr[left] = tmp;


	return partision(arr, left, right);
}

int qsort(int arr[], int left, int right, int n)
{
	if (left == right)
	{
		return arr[left];
	}


	int i = RandomPartision(arr, left, right);


	int k = i - left + 1;


	if (k == n)
	{
		return arr[i];
	}
	else if (k > n)
	{
		return qsort(arr, left, i-1, n);
	}
	else
	{
		return qsort(arr, i+1, right, n-k);
	}


}


int main()
{
	int arr[] = {1, 1, 5, 3, 2, 2, 3, 10, 19};
	int arrLen = sizeof(arr)/sizeof(arr[0]);
	int res = qsort(arr, 0, arrLen - 1, 4);
	printf("%d ", res);
	return 0;
}