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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 on Problem 3292 and last updated at 2015-01-11 10:25:26
//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);
		}
	}
}

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