算法导论9.23习题解答(寻找第i小的数)

CLRS 9.2-3 :

写出RANDOMIZED-SELECT的一个迭代版本

在这里，顺便实现在数组内寻找第i小的数值。

#include <iostream>
#include <time.h>
using namespace std;
//随机化分割
int randomized_partition(int* a, int p, int r);
int randomized_select(int* a, int p, int r, int i);
int main()
{
	int arr[10] = {4, 34, 21, 8, 3, 10, 453, 32, 1, 400};
	int n;
	while(true)
	{
		cout<<"输入第n小的数："<<endl;
		cin>>n;
		cout<<randomized_select(arr, 0, 9, n - 1)<<endl;
	}
	return 0;
}
//下标为[p, r]之间的元素
int randomized_partition(int* a, int p, int r)
{
	srand(time(NULL));
	int q = rand()%(r - p + 1) + p;
	int temp = a[q];
	a[q] = a[r];
	a[r] = temp;
	int j = p;
	for(int i = p; i < r; i++)
	{
		if(a[i] < a[r])
		{
			if(i != j)
			{
				int temp2 = a[i];
				a[i] = a[j];
				a[j] = temp2;
			}
			j++;
		}
	}
	
	temp = a[j];
	a[j] = a[r];
	a[r] = temp;
	return j;
}
//迭代版本
int randomized_select(int* a, int p, int r, int i)
{
	int q = randomized_partition(a, p, r);
	while(p != r)
	{
		if(i == q)
			return a[q];
		else if(i < q)
		{
			r = q - 1;
			q = randomized_partition(a, p, r);
		}
		else
		{
			p = q + 1;
			q = randomized_partition(a, p, r);
		}
	}
	return a[p];
}