[POJ268] Prime Distance(素数筛）

/*
 * 二次筛素数
 * POJ268————Prime Distance（数论，素数筛）
 */


#include<cstdio>
#include<vector>

using namespace std;

const int maxn = 1000005;

typedef long long LL;

bool is_prime_small[maxn];
bool is_prime[maxn];

vector <int> res;

int main() {
    LL l,u;
    while(scanf("%lld%lld",&l,&u) != EOF) {
        for(LL i = 0;i*i < u;i++) {
            is_prime_small[i] = true;
        }
        for(LL i = 0;i <= u-l;i++) {
            is_prime[i] = true;
        }
        is_prime_small[1] = false;
        if(l == 1)
            is_prime[0] = false;
        for(LL i = 2;i*i <= u;i++) {
            if(is_prime_small[i]) {
                for(LL j = 2*i;j*j <= u;j+=i) {
                    is_prime_small[j] = false;
                }
                for(LL j = max(2LL,(l+i-1)/i) * i;j <= u;j+=i) {
                    is_prime[j-l] = false;
                }
            }
        }
        res.clear();
        for(LL i = l;i <= u;i++) {
            if(is_prime[i - l]) {
                res.push_back(i);
            }
        }
        if(res.size() <= 1) {
            printf("There are no adjacent primes.
");
            continue;
        }
        //避免这样写，可能会导致某些变量没有被赋值
//        LL maxx = res[1] - res[0],b1,b2;
//        LL minn = res[1] - res[0],a1,a2;
        LL b1 = res[0],b2 = res[1],maxx = b2 - b1;
        LL a1 = res[0],a2 = res[1],minn = a2 - a1;
        for(LL i = 2;i < res.size();i++) {
            if(maxx < res[i] - res[i-1]) {
                maxx = res[i] - res[i - 1];
                b2 = res[i];
                b1 = res[i-1];
            } else if(minn > res[i] - res[i-1]) {
                minn = res[i] - res[i-1];
                a2 = res[i];
                a1 = res[i-1];
            }
        }
        printf("%lld,%lld are closest, %lld,%lld are most distant.
",a1,a2,b1,b2);
    }
    return 0;
}