C++算法导论第九章O(n)期望选择序列第i小的数字

#include<iostream>
#include<vector>
#include<algorithm>
#include<time.h>
using namespace std;
int randomized_partition(vector<int>& vec, int le, int ri)
{
	if (le == ri)
	{
		return le;
	}
	srand(time(NULL));
	int _rand = le + rand() % (ri - le);
	int X = vec[_rand];	//基准数
	int i = le - 1, j = le;
	swap(vec[_rand], vec[ri]);
	while (j < ri)
	{
		if (vec[j] < X)
		{
			++i;
			swap(vec[i], vec[j]);
		}
		++j;
	}
	swap(vec[i + 1], vec[j]);
	return i + 1;
}

int randomized_select(vector<int>& vec, int le, int ri, int i)
//选择[le,ri]中第i小的元素,O(n)时间期望，i不是索引值
{
	if (le == ri)
	{
		return vec[le];
	}
	int mi = randomized_partition(vec, le, ri);
	int interval = mi - le + 1;
	if (i < interval)
	{
		return randomized_select(vec, le, mi - 1, i);
	}
	else
	{
		if (i == interval)
		{
			return vec[mi];
		}
		else
		{
			return randomized_select(vec, mi + 1, ri, i - interval);
		}
	}
}

int main()
{
	srand(time(NULL));
	vector<int>vec(100);
	for (int i = 0; i < 100; ++i)
	{
		vec[i] = rand() % 100;
	}
	cout << randomized_select(vec, 0, 99, 6) << endl;
	sort(vec.begin(), vec.end());
	for (int i = 0; i < 100; ++i)
	{
		cout << vec[i] << "  ";
	}
	system("pause");
}