判断一个点是否在三角形内

#include <iostream>
#include <cstring>
#include <queue>
#include <cmath>
#include <cstdio>
#include <algorithm>
using namespace std;
const double eps = 1e-8;
struct point
{
    double x,y;
};
double solve(point a,point b,point c)
{
    point A;
    A.x = b.x-a.x;
    A.y = b.y-a.y;
    B.x = c.x-a.x;
    B.y = c.y-a.y;
    return (A.x*B.y-B.x*A.y)/2.0;
}

int main()
{
    point A,B,C,P;
    cin>>A.x>>A.y;
    cin>>B.x>>B.y;
    cin>>C.x>>C.y;
    cin>>P.x>>P.y;
    double sum = solve(A,B,C);
    double k=0;
    k + = solve(A,B,P);
    k + = solve(B,C,P);
    k + = solve(A,C,P);
    if((k-sum)>eps) cout<<"在三角形外"<<endl;
    else cout<<"在三角形内"<<endl;
    return 0;
}