速い列の思想を使ってn番目の大きい値を求めます

#include "iostream"
#include "vector"
#include <ctime>
#include <cstdlib>
using namespace std;
template <typename T>
int partition(vector<T> &a,int low,int high){
	T temp = a[low];
	while(low < high){
		while((low < high) && (a[high] > temp)) --high;
		if(low < high) a[low++] = a[high];
		while((low < high) && a[low] < temp) ++low;
		if(low < high) a[high--] = a[low];
	}
	a[low] = temp;
	return low;
}
template <typename T>
T nthElement(vector<T> &a,int low,int high,int n){
	if(low > high || n < 1 || n > high - low + 1) {
		cout<<"error"<<endl;
		system("pause");
	}
	int pos;
	while(true){
		pos = partition(a,low,high);
		if(low + n - 1 == pos) return a[pos];
		else if(low + n -1 < pos) high = pos - 1;
		else {
			n = n - (pos - low + 1);// pos     pos - low + 1    n    
			low = pos + 1;
		}
	}
}
int main(int argc, char const *argv[])
{
	int num = 6;
	srand(time(0));
	std::vector<int> a(num);
	for (int i = 0; i < num; ++i)
	{
		       //seed
		int rand_num = rand()%100;
		a[i] = rand_num;
		cout<<rand_num<<"\t";
	}
	cout<<endl;
	cout<<nthElement(a,0,num - 1,3)<<endl;
}