ZOJ - 3609 (逆元、扩展欧几里得板子)

#include<cstdio>
using namespace std;
typedef long long ll;

ll extend_gcd(ll a, ll b, ll &x, ll &y){
	if(!a&&!b) return -1;
	if(b==0){
		x=1; y=0; return a;
	}
	ll d=extend_gcd(b, a%b, y, x);
	y-=(a/b)*x;
	//printf("%lld %lld %lld %lld\n", a, b, x, y);
	return d;
}

ll mod_reverse(ll a, ll mod){
	ll x, y;
	ll d=extend_gcd(a, mod, x, y);
	
	if(d==1) return (x%mod+mod)%mod;
	else return  -1;
}

int main(){
	int T;	
	scanf("%d", &T);
	while(T--){
		ll n, m;
		scanf("%lld%lld", &n, &m);
		
		if(m==1)
		{
			printf("1\n"); continue;
		}
		
		ll ans=mod_reverse(n, m);
		
		if(ans==-1)
			printf("Not Exist\n");
		else
			printf("%lld\n", ans);
	}
	return 0;
}