#include "stdafx.h"
#include<iostream>
#include <stdlib.h>
#include <time.h>
using namespace std;#define SB -1int RANDOM(int p, int r)
{srand((unsigned)time(NULL));return (rand() % (r - p + 1)) + p;
}int partition(int a[], int p, int r) {int x, i, t;x = a[r];i = p - 1;t = 0;for (int j = p; j <= r - 1; j++){if (a[j]< x){i = i + 1;int ti;ti = a[i];a[i] = a[j];a[j] = ti;}if (a[j] == x){t = t + 1;int ti;ti = a[i + t];a[i + t] = a[j];a[j] = ti;}}int tii;tii = a[i + t + 1];a[i + 1 + t] = a[r];a[r] = tii;return i + 1;
}int random_partion(int a[], int p, int r)
{int i = RANDOM(p, r);int tii;tii = a[i];a[i] = a[r];a[r] = tii;return partition(a, p, r);
}/*int random_select(int a[], int p, int r, int i)  //随机选择算法递归实现
{if (p == r)return a[p];int q;q = random_partion(a, p, r);int k = q - p + 1;if (i == k)return a[q];else if (i < k)return random_select(a, p, q - 1, i);elsereturn random_select(a, q+1, r, i-k);
}*/int random_select2(int a[], int p, int r, int i)  //随机选择算法循环实现
{if (p == r)return a[p];int q;int head = p, end = r;q = random_partion(a, head, end);while (i != q) {if (i < q) {end = q - 1;if (end > r)end = r;q = random_partion(a, head, end);}else {head = q + 1;if (head > r)head = r;q = random_partion(a, head, end);}}return a[q];
}int main()
{int a[] = { SB ,3 ,2 ,4 ,5 ,6 ,7,8,9,10,11,12,13};   //SB为哨兵，不包括在选择项cout << random_select2(a, 1, 12, 7);while (1);return 0;
}