HDU 2701 Max Factor

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Max Factor

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 8635    Accepted Submission(s): 2817

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

题意就是给你 n 个数，求出这些数中，因数为素数且最大的数。若都是素数，则需要输出最大的数。

素数打表，若i是素数，则a[i]=1。为了防止都是素数，先进行排序。然后倒序查找最大的质因数。

代码实现：

#include<iostream>#include<cstring>#include<algorithm>using namespace std;int a[50000];int main(){memset(a,1,sizeof(a));a[0]=0;a[1]=0;a[1]=0;a[2]=1;for(int i=2;i<=20005;i++){if(a[i]){for(int j=i+i;j<=20005;j=j+i){a[j]=0;}}}int n,i,j,map[5005],sum,max,flag,k;while(cin>>n){max=-1;for(i=0;i<n;i++)cin>>map[i];sort(map,map+n);for(i=0;i<n;i++){for(j=map[i];j>=1;j--){k=1;if(a[j]){if(map[i]%j==0){sum=j;k=0;}}if(k==0)break;}if(sum>max){max=sum;flag=i;}}if(max==1)cout<<map[n-1]<<endl;elsecout<<map[flag]<<endl;}return 0;}