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hdu 2710 Max Factor（找最大素数）

2024-07-18 16:07:04   219人阅读

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=2710


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
4
36
38
40
42
 

Sample Output
38
 

Source
USACO 2005 October Bronze


题意：
求给出数据含有最大质数的数，如：36 38 40 42 即19为38的最大质数，也是4个数中最大的质数。

所以输出包含最大质数的数38，还要题目最后说如果有多个，要输出最前面的那个数。

思路：

先打表出所有的素数，然后再寻找即可！

代码如下：

#include <cstdio>
#include <cstring>
const int MAXN = 20017;
int s[MAXN];
int main()
{
    int n,m;
    memset(s,0,sizeof(s));
    s[1]=1;//此题1也是素数
    for(int i = 2; i < MAXN; i++)//筛选所有范围内的素数
    {
        if(s[i] == 0)//没有被更新过
            for(int j = i; j < MAXN; j+=i)
            {
                s[j]=i;
            }
    }
    while(~scanf("%d",&n))
    {
        int ans;
        int maxx = -1;
        for(int i = 0; i < n; i++)
        {
            scanf("%d",&m);
            if(s[m] > maxx)
            {
                maxx = s[m];
                ans = m;
            }
        }
        printf("%d\n",ans);
    }
    return 0;
}



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