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 题意：给你n个数，让你求出其中质因数最大的数思路：这是筛素数的思想，在筛素数的时候，记录每个数对应的最大质因数，这样在访问该数的时候可以以o（1）的复杂度找到它所对应的最大质因数。再加上贪心的想法，在每次访问的时候找到当前最大质因数，并且记录该数。 代码如下：
#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
using namespace std;
const int N=20010;
bool a[N];
int ans[N];
void isprime() //作用：找到每个数所对应的最大素数
{
int k=0;
memset(a,true,sizeof(a));
a[0]=a[1]=false;
for(int i=2;i<N;i++)
{
if(a[i])
{
ans[i]=i;
for(int j=i*2;j<N;j+=i)
ans[j]=i；
a[j]=false;
}
}
}
int main()
{
isprime();
int m;
while(~scanf("%d",&m))
{
int maxn=-N;
int x=-N;
while(m--)
{
int a;
scanf("%d",&a);
if(ans[a]>maxn)
{
maxn=ans[a];
x=a;
}
}
printf("%d\n",x);
}
return 0;
}