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:找到数内最大的质因子,输出这个数
代码如下:
#include<iostream>
#include<cstdio>
#include<cstring>
#include <cmath>
#include <algorithm>
using namespace std;
#define n 20017
int pri[n];
void prime()
{
pri[1] = 1;
for(int i = 2;i <= n;i ++)
{
if(!pri[i])
{
for(int j = i;j <= n;j = j + i)
pri[j] = i;
}
}
}
int main()
{
prime();
int t,x,max,maxn;
while(~scanf("%d",&t))
{
max = 0;
maxn = -1;
for(int i = 0;i < t;i ++)
{
scanf("%d",&x);
if(pri[x] > max)
{
max =pri[x];
maxn = x;
}
}
printf("%d
",maxn);
}
return 0;
}