a121: 質數又來囉 FYI: https://yunlinsong.blogspot.com/2015/02/blog-post.html

a121.cpp

#include <cstdio>
#include <cmath>

int main(int argc, char** argv) {
	bool isPrime;
	int primeCnt;
	unsigned long a, b;

	//while(cin >> a >> b)
	while(scanf("%d%d", &a, &b) != EOF)  
	{
		//printf("%d %d\n", a, b);
		primeCnt = 0;
		
		for(unsigned long i = a; i <= b; i++)
		{
			isPrime = true;
			if( i == 2 )
				primeCnt++;
			
			else if( i == 3 )
				primeCnt++;
			
			else if( i == 5 )
				primeCnt++;
			
			else if( i == 7 )
				primeCnt++;
			
			if( i == 1 || i % 2 == 0 || i % 3 == 0 || i % 5 == 0 || i % 7 == 0)
				continue;
			
			unsigned long sqrt_i = sqrt(i);
			for(unsigned long k = 9; k <= sqrt_i; k += 2)
			{
				if(k % 2 == 0 || k % 3 == 0 || k % 5 == 0)
					continue;
				
				if( i % k == 0)
				{
					isPrime = false;
					break;
				}
			}
			
			if(isPrime == true)
			{
				//printf("Prime:%ld\n", i);
				primeCnt++;
			}
		}
		
		
		//cout << primeCnt << endl;
		printf("%d\n", primeCnt);
	}

	return 0;
}