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Re:贴Java代码。筛选后排序+二分查找 ,带注释

Posted by Doffery at 2015-02-24 18:31:08 on Problem 3292
In Reply To:贴Java代码。筛选后排序+二分查找 ,带注释 Posted by:haqishen at 2015-01-11 10:21:02
> //筛选后排序+二分查找
> import java.util.*;
> 
> public class Main{
> 	static final int n = 250001;
> 
> //      nums用于进行筛选,Hprimes用于保存生成出来的Hprime
> 	static boolean nums[] = new boolean[n];
> 	static ArrayList<Integer> Hprimes = new ArrayList<Integer>();
> 
> //      用筛选法生成Hprimes
> 	static void generate_Hprimes(){
> 		for(int i = 0; i < n; i++)
> 			nums[i] = true;
> 		nums[0] = nums[1] = false;
> 		
> 		for(int i = 1; i < n; i++){
> //                      第一个Hnumber
> 			int n1 = 4*i + 1;
> 			for(int j = 1; j <= i; j++){
> //                              第二个Hnumber
> 				int n2 = 4*j + 1;
> //                              两者相乘得到一个由2个Hnum构成的Hnum
> 				int n3 = n1 * n2;
> 				if(n3 > 4*n) break;
> //                              如果这个数还没有被筛选掉,则说明这个数是Hprime
> 				if(nums[n3/4]){
> //                                      将这个数放入Hprimes
> 					Hprimes.add(n3);
> //                                      进行筛选
> 					for(int p = n3 * 1; p < 4*n; p += 4*n3){
> 						nums[p/4] = false;
> 					}
> 				}
> 			}
> 		}
> 	}
> 	public static void main(String[] args) {
> 		Scanner s = new Scanner(System.in);
> 		
> //              生成Hprimes,然后升序排序
> 		generate_Hprimes();
> 		Collections.sort(Hprimes);
> 		
> 		while(true)
> 		{
> 			int h = s.nextInt();
> 			if(h==0) break;
> 			
> //                      对于输入的数字进行二分查找,然后下标就是答案
> 			int l = 0, r = Hprimes.size()-1;
> 			while(l<=r){
> 				int m = (l+r)/2;
> 				if(Hprimes.get(m) > h){
> 					r = m - 1;
> 				}else{
> 					l = m + 1;
> 				}
> 			}
> 			System.out.println(h+" "+l);
> 		}
> 	}
> }

不是prime,是semi

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