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

4 36 38 40 42

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; }