POJ 2478 欧拉函数（欧拉筛法） HDU 1576 逆元求法

#include <stdio.h>
#include <string.h>
#include <iostream>
using namespace std;
typedef long long LL;
const int N = 1e6 + 500;

int phi[N];
LL pre[N];
int cnt = 0;
void init()
{
    for(int i = 0; i < N; i++)
        phi[i] = i;
    for(int i = 2; i < N; i++)
    {
        if(phi[i] == i)
        {
            phi[i] = i - 1;
            for(int j = 2 * i; j < N; j += i)
                phi[j] = phi[j] / i * (i - 1);
        }
    }
    phi[1] = 0;
    memset(pre, 0, sizeof(pre));
    for(int i = 2; i < N; i++)
        pre[i] = pre[i-1] + phi[i];
}

int main()
{
    int n;
    init();
    while(scanf("%d", &n) , n)
        printf("%I64d\n", pre[n]);
    return 0;
}

#include <stdio.h>
#include <string.h>
#include <iostream>
using namespace std;
typedef long long LL;

int T;
LL a, b;

void extend_gcd(LL a, LL b, LL &x, LL &y, LL &d)
{
    if(b == 0)
    {
        x = 1;
        y = 0;
        d = a;
    }
    else
    {
        extend_gcd(b, a%b, y, x, d);
        y -= (a/b) * x;
    }
}

int main()
{
    scanf("%d", &T);
    while(T--)
    {
        scanf("%lld%lld", &a, &b);
        LL x , y, d;
        extend_gcd(b, 9973, x, y, d);
        x = (x + 9973) % 9973;
        printf("%lld\n", (a*x) % 9973);
    }
    return 0;
}