package math;
import java.util.Scanner;
import java.util.Random;
public class RandomSelect {
  public static void swap(int x, int y) {
    int temp = x;
    x = y;
    y = temp;
  }
  public int Random (int x, int y) {
    Random random = new Random();
    int num = random.nextInt(y)%(y - x + 1) + x;
    return num;
  }
  public int partition(int[] list, int low, int high) {
    int tmp = list[low];  //数组的第一个作为中轴
    while (low < high) {
      while (low < high && list[high] > tmp) {
        high--;
      }
      list[low] = list[high];  //比中轴小的记录移到低端
      while (low < high && list[low] < tmp) {
        low++;
      }
      list[high] = list[low];  //比中轴大的记录移到高端
    }
    list[low] = tmp;       //中轴记录到尾
    return low;          //返回中轴的位置
  }
  public int RandomizedPartition (int[] arrays, int left, int right) {
    int i = Random(left, right);
    swap(arrays[i], arrays[left]);
    return partition(arrays, left, right);
  }
  public int RandomizedSelect(int[] arrays, int left, int right, int k) {
    if(left == right ) {
      return arrays[left];
    }
    int i = RandomizedPartition(arrays, left, right);
    int j = i - left + 1;
    if(k <= j) {
      return RandomizedSelect(arrays,left, i,k) ;
    }
    else {
      return RandomizedSelect(arrays,i+1,right,k-j);
    }
  }
  public static void main(String args[]) {
    System.out.println("编程素材网测试结果：");
    int[] a = {7,5,3,4,8,6,9,1,2};
    for (int i = 0; i < 9; i ++) {
      System.out.print(a[i]+ " ");
    }
    System.out.println();
    RandomSelect r = new RandomSelect();
    System.out.println("你要查询的元素是数组中第几小的？");
    @SuppressWarnings("resource")
 Scanner sc = new Scanner(System.in);
    int m = sc.nextInt();
    int n = r.RandomizedSelect(a,0,8,m);
    System.out.println("这个数组中第" + m + "小的元素是："+ n);
  }
}