两条线段的位置 左旋/右旋

    AB 到 BC 右旋 ABxBC<0
    AB 到 BC 左旋 ABxBC>0

[ x为叉积 AB=(xb-xa,yb-ya) ]

#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cstring>
#include <time.h>
#include <string>
#include <set>
#include <map>
#include <list>
#include <stack>
#include <queue>
#include <vector>
#include <bitset>
#include <ext/rope>
#include <algorithm>
#include <iostream>
using namespace std;
#define ll long long
#define minv 1e-6
#define inf 1e9
#define pi 3.1415926536
#define nl 2.7182818284
const ll mod=1e9+7;//998244353
const int maxn=1e5+10;

struct point
{
    ll x,y;
}a,b,c;

ll compare(point u,point v)
{
    return u.x*v.y-v.x*u.y;
}

ll cross(point u,point v,point w)
{
    return compare({v.x-u.x,v.y-u.y},{w.x-v.x,w.y-v.y});
}

int main()
{
    /**
    input:
    A:(xa,ya)
    B:(xb,yb)
    C:(xc,yc)

    AB 到 BC 右旋 value<0
    AB 到 BC 左旋 value>0
    **/
    ll xa,ya,xb,yb,xc,yc,v;
    scanf("%lld%lld",&xa,&ya);
    a={xa,ya};
    scanf("%lld%lld",&xb,&yb);
    b={xb,yb};
    scanf("%lld%lld",&xc,&yc);
    c={xc,yc};

    v=cross(a,b,c);
    if (v<0)
        printf("%d",-1);
    else if (v>0)
        printf("%d",1);
    else
        printf("%d",0);
    return 0;
}
/*
2 1
1 0
0 1
-1

2 1
1 0
0 -2
1

2 1
1 0
0 -1
0

------

0 1
1 0
2 1
1

0 -2
1 0
2 1
-1

0 -1
1 0
2 1
0

*/