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
哎~又一个英文题~
题意：
输入一个正整数n，然后输入n个正整数（1<=a[i]<=20000），要你求这n个数里哪个数的最大素因数（即能被该数整除(包括这个数本身！)的最大素数）最大，然后输出这个数。若有两个数的最大素因数相同，则输出前面那个。
用到了素数快速筛选~不然会超时的~
import java.util.Arrays; import java.util.Scanner; /** * @author 陈浩翔 */ public class Main{ static boolean db[] = new boolean[20005]; public static void main(String[] args) { dabiao(); Scanner sc = new Scanner(System.in); while(sc.hasNext()){ int n = sc.nextInt(); int a[] = new int[n]; int prime[] = new int[n]; for(int i=0;i<n;i++){ a[i]=sc.nextInt(); for(int k=1;k<=a[i];k++){ if(db[k]&&a[i]%k==0){ prime[i]=k; } } } int max=prime[0]; int con=0; for(int i=1;i<n;i++){ if(prime[i]>max){ max=prime[i]; con=i; } } System.out.println(a[con]); } } private static void dabiao() { Arrays.fill(db, true); for(int i=2;i<=Math.sqrt( db.length);i++){ for(int j=i+i;j<db.length;j+=i){ if(db[j]==true){ db[j]=false; } } } } }
