HDU 2161 Primes

http://acm.hdu.edu.cn/showproblem.php?pid=2161

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

Sample Output

1: no

2: no

3: yes

4: no

5: yes

6: yes

时间复杂度：$O(sqrt{N})$

代码：

#include <bits/stdc++.h>
using namespace std;

int N;

int prime(int x) {
    for(int i = 2; i * i <= x; i ++) {
        if(x % i == 0)
            return 0;
    }
    return 1;
}

int main() {
    int Case = 1;
    while(~scanf("%d", &N)) {
        if( N <= 0) break;
        printf("%d: ", Case ++);
        if(N == 1 || N == 2)
            printf("no
");

        else {
             if(prime(N))
                printf("yes
");
            else
                printf("no
");
        }
    }
    return 0;
}