ZOJ 3609 Modular Inverse

ZOJ 3609

#include <cstdio>
#include <cmath>
using namespace std;
int ex_gcd(int a,int b,int &x,int &y)//solve x,y in a*x+b*y=ex_gcd(a,b,x,y)=gcd(a,b);
{
    if(b==0)
    {
        x=1;
        y=0;
        return a;
    }
    int ans=ex_gcd(b,a%b,x,y);
    int tmp=x;
    x=y;
    y=tmp-a/b*y;
    return ans;
    //x = x0 + (b/gcd)*t
    //y = y0 – (a/gcd)*t
    //we can solve x,y in a*x+b*y=c <=> c%gcd(a,b)==0 
}

int get(int a,int m)//get x in a*x=1(mod m)
{
    int x,y;
    int gcd=ex_gcd(a,m,x,y);
    if(1%gcd!=0)
        return -1;//error
    x*=1/gcd;
    m=abs(m);
    int ans=x%m;
    if(ans<=0)
        ans+=m;
    return ans;
}

int main()
{
    int a,b;
    int t;
    scanf("%d",&t);
    while(t--)
    {
        scanf("%d%d",&a,&b);
        int ans=get(a,b);
        if(ans==-1)
            printf("Not Exist
");
        else
            printf("%d
",ans);
        // scanf("%d%d",&a,&b);
        // int gcd=ex_gcd(a,b,x,y);
        // printf("gcd(%d,%d)=%d
",a,b,gcd);
        // printf("x:%d,y:%d
",x,y);
        // printf("%d*%d+%d*%d=%d
",a,x,b,y,gcd);    
    }
    
    return 0;
}