HDU 2161 Primes

Primes

Time Limit: 1000/1000 MS (Java/Others) Memory Limit: 32768/32768 K (Java/Others)

Problem Description

Write a program to read in a list of integers and determine whether or not each number is prime. A number, n, is prime if its only divisors are 1 and n. For this problem, the numbers 1 and 2 are not considered primes.

Input

Each input line contains a single integer. The list of integers is terminated with a number<= 0. You may assume that the input contains at most 250 numbers and each number is less than or equal to 16000.

Output

The output should consists of one line for every number, where each line first lists the problem number, followed by a colon and space, followed by “yes” or “no”.

Sample Input

1
2
3
4
5
17
0

Sample Output

1: no
2: no
3: yes
4: no
5: yes
6: yes

题意：

判断是否是素数，2不是素数（不知道这题为什么是这样得，其它地方2是素数）

思路：

可以用普通得判断方法，也可以用筛法。

#include 
#include 
#include 
using namespace std;
const int maxn = 16010;
bool book[maxn] = {false};
int prime[maxn] = {0};
int cnt = 0;
void IsPrime() {
    for (int i = 2; i <= maxn; i++) {
        if (book[i] == false) prime[cnt++] = i;
        for (int j = 0; j < cnt && i * prime[j] <= maxn; j++) {
            book[i * prime[j]] = true;
            if (i % prime[j] == 0) break;
        }
    }
}
int main() {
    IsPrime();
    book[2] = book[1] = true;
    int n, Case = 0;
    while (scanf("%d", &n) != EOF && n > 0) {
        if (book[n]) printf("%d: no\n", ++Case);
        else printf("%d: yes\n", ++Case);
    }
    return 0;
}