hdu 2710 Max Factor

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

436384042

 

Sample Output

38

 

Source

USACO 2005 October Bronze

题意：

求给出数据含有最大质数的数，然后输出包含最大质数的数，如果有多个，要输出最前面的那个数。

//以前用素筛只是简单的拿来快速判断一个数是不是素数，没想到还可以这样用，通过素筛，把每一个数的最大质数找出来

#include <iostream>
#include <bits/stdc++.h>

using namespace std;

int a[20050];

int main()
{
    memset(a,0,sizeof(a));
    a[1]=1;
    for(int i=2; i<20010; i++)
    {
        if(a[i]==0)
        {
            for(int j=i; j<20010; j+=i)
                a[j]=i;
        }
    }
    int n,x,mx=-1,res;
    while(cin>>n)
    {
    mx=-1;
        for(int i=0; i<n; i++)
        {
            scanf("%d",&x);
            if(a[x]>mx)
            {
                mx=a[x];
                res=x;
            }
        }
        cout<<res<<endl;
    }
    return 0;
}