Max Factor（技巧题）

Max Factor

Time Limit: 1000MS Memory Limit: 65536KTotal Submissions: 6649 Accepted: 2609

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

Hint

OUTPUT DETAILS:
19 is a prime factor of 38. No other input number has a larger prime factor.

Source

USACO 2005 October Bronze

题意：

从一串数字中找到有最大素数因子的数输出。

思路：

在判断是否为素数时，改变，用点小技巧，另不是素数的等于他的一个最大的素数因子。

代码：

#include <iostream>#include <cstring>#include <string>#include <stdio.h>using namespace std;int a[20055];int maxx,num;int main(){    int n,m;    memset(a,0,sizeof(a));    a[1]=1;    for(int i=2; i<20055;i++)    {        if(a[i]==0)            for(int j=i;j<20055;j+=i)            {                a[j]=i;  //关键点            }    }    while(~scanf("%d",&n))    {        maxx=-1;        for(int i=0;i<n;i++)        {            scanf("%d",&m);            if(a[m]>maxx)            {                maxx=a[m];                num=m;            }        }        printf("%d\n",num);    }    return 0;}