HDU 2710 Max Factor （素数筛选）

Max Factor

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 6348    Accepted Submission(s): 2087

Problem Description

To improve the organization of his farm, Farmer John labels each of his N (1 <= N <= 5,000) cows with a distinct serial number in the range 1..20,000. Unfortunately, he is unaware that the cows interpret some serial numbers as better than others. In particular, a cow whose serial number has the highest prime factor enjoys the highest social standing among all the other cows.

(Recall that a prime number is just a number that has no divisors except for 1 and itself. The number 7 is prime while the number 6, being divisible by 2 and 3, is not).

Given a set of N (1 <= N <= 5,000) serial numbers in the range 1..20,000, determine the one that has the largest prime factor.

Input

* Line 1: A single integer, N

* Lines 2..N+1: The serial numbers to be tested, one per line

 

Output

* Line 1: The integer with the largest prime factor. If there are more than one, output the one that appears earliest in the input file.

Sample Input

   
   
   
   

    
    
    
    4 36 38 40 42
   
   
   
   

Sample Output

   
   
   
   

    
    
    
    38
   
   
   
   

 

Source

USACO 2005 October Bronze

题解：素数筛选.....暴力一波

AC代码：

#include<iostream>   
#include<cstdlib>  
#include<cstdio>  
#include<cmath>  
#include<cstring>  
#include<string>  
#include<cstdlib>  
#include<iomanip>  
#include<vector>  
#include<list>  
#include<map>  
#include<queue>
#include<algorithm>  
typedef long long LL;  
using namespace std; 
const int maxn=20005;
int s[maxn];
int main()
{
    int n,m,MAX,p;
    memset(s,0,sizeof(s));
    s[1]=1;
    for(int i=2;i<maxn;i++)//筛选所有范围内的最大素数因子
    {
    	if(s[i]==0)
    	for(int j=i;j<maxn;j+=i)
    	{
    		s[j]=i;
		}
	}
	while(cin>>n)
	{
		MAX=-1;
		while(n--)
		{
			cin>>m;
			if(s[m]>MAX)
			{
				MAX=s[m];
				p=m;
			}
		}
		cout<<p<<endl;
	}
    return 0;
}