HDU - 2710 - Max Factor【筛法求素数】题解

电玩女神 2021-07-24 23:07 409阅读 0赞

### 目录 ###

*   *   *  1.题目
         *  2.代码

### 1.题目 ###

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

### 2.代码 ###

#include<iostream>
    #include<cstdio>
    using namespace std;
    const int maxn=20010;
    int prime[maxn];
    void isPrime()
    { 
    	prime[1]=1;//特殊 筛法素数的改版 
    	for(int i=2;i<maxn;i++)
    	{ 
    		if(prime[i]==0)
    		{  
    		  prime[i]=i;
    		  for(int j=i+i;j<maxn;j+=i)
    		   prime[j]=i;//特殊 
    		} 		
    	}
    }
    int main()
    { 
       isPrime();
       int t,n,ans;
       while(cin>>t)
       { 
       	 int max=0;
         for(int i=0;i<t;i++)
         { 
       	   cin>>n;
       	   if(prime[n]>max)
       	   { 
    	      max=prime[n];
    		  ans=n;	  
    	   }
         }
         cout<<ans<<endl;
       }
       return 0;	
    }