欧几里得(gcd) + 拓展欧几里得(ext_gxd)

1. 欧几里得算法

//求最大公约数和最小公倍数

int gcb(int a, int b){    return b == 0 ? a : gcb(a, a%b);}

2.拓展欧几里得

求解线性方程组 ax + by + c = 0

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

//计算乘法逆元

int extgcd(ll a, ll b, ll &x, ll &y){    if(b == 0){        x = 1;        y = 0;        return a;    }else{        ll ans = extgcd(b, a%b, x, y);        ll t = x;        x = y;        y = t - a/b*y;        return ans;    }}//计算乘法逆元int cal(ll a, ll b, ll d){    ll x, y;    ll gcd = extgcd(a, b, x, y);    if(d % gcd != 0) return -1;    x = x*d/gcd;    b = b/gcd;    if(b < 0) b = -b;    ll ans = x%b;    if(ans < 0) ans += b;    return ans;}