Poj 2388 Who's in the Middle

最新推荐文章于 2022-10-02 20:16:14 发布

weixin_33852020

最新推荐文章于 2022-10-02 20:16:14 发布

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原文链接：http://www.cnblogs.com/mobileliker/p/3937175.html

本文介绍了一个通过快速选择算法解决的问题：在一个奇数数量的奶牛群中找到产奶量中位数的奶牛。文章提供了完整的C++实现代码，并解释了如何使用随机化分区函数来提高算法效率。

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1.Link：

http://poj.org/problem?id=2388

2.Content：

Who's in the Middle

Time Limit: 1000MS Memory Limit: 65536K
Total Submissions: 31854 Accepted: 18541

Description

FJ is surveying his herd to find the most average cow. He wants to know how much milk this 'median' cow gives: half of the cows give as much or more than the median; half give as much or less.

Given an odd number of cows N (1 <= N < 10,000) and their milk output (1..1,000,000), find the median amount of milk given such that at least half the cows give the same amount of milk or more and at least half give the same or less.

Input

* Line 1: A single integer N

* Lines 2..N+1: Each line contains a single integer that is the milk output of one cow.

Output

* Line 1: A single integer that is the median milk output.

Sample Input
5
2
4
1
3
5
Sample Output
3
Hint

INPUT DETAILS:

Five cows with milk outputs of 1..5

OUTPUT DETAILS:

1 and 2 are below 3; 4 and 5 are above 3.

Source

USACO 2004 November

3.Method：

4.Code：

 1 #include "iostream"
 2 #include "stdlib.h"
 3 #include "stdio.h"
 4 #include <time.h>
 5 #define N 10002
 6 using namespace std;
 7 int a[N];
 8 int Partition(int a[],int p,int r)
 9 {
10     int tmp = a[p];
11     while(p<r)
12     {
13         while(p<r && a[r]>=tmp) r--;
14         a[p]=a[r];
15         while(p<r && a[p]<=tmp) p++;
16         a[r]=a[p];
17     }
18     a[p]=tmp;
19     return p;
20 }
21 int RandomizePartition(int a[],int p,int r)
22 {
23     int i = rand()%(r-p+1)+p;
24     int tmp=a[i];
25     a[i]=a[p];
26     a[p]=tmp;
27     return Partition(a,p,r);
28 }
29 
30 int RandomizedSelect(int a[],int p,int r,int k)
31 {
32     if(p==r) return a[p];
33     int i = RandomizePartition(a,p,r),j = i-p+1;
34     if(k<=j) return RandomizedSelect(a,p,i,k);
35     else return RandomizedSelect(a,i+1,r,k-j);
36 }
37 int main()
38 {
39     srand(time(NULL));//初始化随机数生成器
40     int n;
41     int i;
42     scanf("%d",&n);
43     for(i=0;i<n;i++) scanf("%d",&a[i]);
44     //qsort(a,0,n-1);
45     //printf("%d\n",a[n/2]);
46     printf("%d\n",RandomizedSelect(a,0,n-1,(n+1)/2));
47     system("pause");
48     return 0;
49 }

5.Reference：

转载于:https://www.cnblogs.com/mobileliker/p/3937175.html

Time Limit: 1000MS		Memory Limit: 65536K
Total Submissions: 31854		Accepted: 18541