判断点是否在三角形内

求面积根据叉乘：不懂可以先看一下我的博客里计算机几何，叉乘2016-11-1016:11:01

#include <iostream>
#include <algorithm>
#include <cmath>
using namespace std;

struct point {
    double x, y;
};

struct v {
    point start, end;
};


double dotProduct(v v1, v  v2) {
    return (v1.end.x - v1.start.x)*(v2.end.x - v2.start.x) + (v1.end.y - v1.start.y)*(v2.end.y - v2.start.y);
}
double crossProduct(v v1, v v2) {
    return (v1.end.x - v1.start.x)*(v2.end.y - v2.start.y) - (v2.end.x - v2.start.y)*(v1.end.y - v1.start.y);
}

bool inTriangle(point a,point b,point c, point p) {
    v ab, ac, bc;
    ab.start = a, ab.end = b;
    ac.start = a, ac.end = c;
    bc.start = b, bc.end = c;
    double Sabc = fabs(crossProduct(ab, ac));

    v pb;
    pb.start = p, pb.end= b;
    double Sabp = fabs(crossProduct(ab, pb));

    v pc;
    pc.start = p, pc.end = c;
    double Sacp = fabs(crossProduct(ac, pc));

    double Sbcp = fabs(crossProduct(bc, pc));

    if (fabs(Sabc - Sabp - Sacp - Sbcp) < 1e-5)
        return true;
    return false;
}


int main() {
    point p1, p2, p3,p0;
    while (cin >> p1.x >> p1.y >> p2.x >> p2.y >> p3.x >> p3.y >> p0.x >> p0.y) {
        cout << inTriangle(p1, p2, p3, p0);
    }
}