poj2689

如何在区间范围以内筛选素数。

#include <cstdio>#include <string.h>#include <cmath>const int MAX_LENGTH = 1000006;const int MAX_PRIME = 50000;bool vis[MAX_PRIME + 2];bool is_prime[MAX_LENGTH];int prime[MAX_PRIME];int prime_number;void getAllPrime() {memset(vis, 0, sizeof(vis));int m = (int)sqrt(MAX_PRIME + 0.5);for (int i = 2; i <= m; i++) {if (!vis[i]) {for (int j = i * i; j <= MAX_PRIME; j += i) {vis[j] = 1;}}}prime_number = 0;for (int i = 2; i <= MAX_PRIME; i++) {if (!vis[i]) {prime[++prime_number] = i;}}}void getSegmentPrime(long long left, long long right) {memset(is_prime, true, sizeof(is_prime));if (left == 1) {is_prime[0] = 0;}for (long long i = 1; i <= prime_number && prime[i] * prime[i] <= right; i++) {long long begin = (left / prime[i]) + (left % prime[i] > 0);if (begin == 1) {begin++;}for (long long j = begin * prime[i]; j <= right; j = j + prime[i]) {is_prime[j - left] = 0;}}}int main() {getAllPrime();long long left, right;while (scanf("%I64d%I64d", &left, &right) != EOF) {getSegmentPrime(left, right);int flag = 0;int first = 1;long long pre, dist_1, dist_2, closet_1, closet_2;long long max_dist = -1;long long min_dist = MAX_LENGTH;for (long long i = left; i <= right; i++) {if (is_prime[i - left]) {if (first != 1) {long long temp = i - pre;flag = 1;if (temp > max_dist) {dist_1 = pre;dist_2 = i;max_dist = temp;}if (temp < min_dist) {closet_1 = pre;closet_2 = i;min_dist = temp;}}first = 0;pre = i;}}if (!flag) {printf("There are no adjacent primes.\n");}else {printf("%I64d,%I64d are closest, %I64d,%I64d are most distant.\n", closet_1, closet_2, dist_1, dist_2);}}return 0;}