HDU2138 How many prime numbers

标签：算法

PS: 素数判定问题。

#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;

long long pow_mod(long long a, long long i, long long n) {
    if(i==0) return 1%n;
    long long tmp = pow_mod(a, i>>1, n); // int tmp. WA.
    tmp = tmp*tmp%n;
    if(i&1) tmp = (long long)tmp*a%n;
    return tmp;
}

bool test(int n, int a, int d) {
    if(n==2) return true;
    if(n==a) return true;
    if((n&1)==0) return false;
    while(!(d&1)) d = d>>1;
    int t = pow_mod(a, d, n);
    while((d!=n-1) && (t!=1) && (t!=n-1)) {
        t = (long long)t*t%n;
        d = d << 1;
    }
    return (t==n-1 || (d&1)==1);
}

bool isPrime(int n) {
    if(n<2) return false;
   // int a[] = {2, 3 , 5, 7, 11, 13, 17};
    int a[] = {2, 3, 5};
    for(int i = 0; i <= 2; i++) {
        if(!test(n, a[i], n-1)) return false;
    }
    return true;
}
int main()
{
    int n, t, cnt;
    while(scanf("%d", &n)!=EOF) {
        int *p = new int[n+5];
        for(int i = 0; i < n; i++) {
            scanf("%d", &t);
            p[i] = t;
        }
        cnt = 0;
        for(int i = 0; i < n; i++) {
            if(isPrime(p[i])) cnt++;
        }
        printf("%d\n", cnt);
    }
    return 0;
}

HDU2138 How many prime numbers

标签：算法

原文地址：http://blog.csdn.net/achiberx/article/details/24652505