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AOJ 0012

A Point in a Triangle | Aizu Online Judge

外積を用いて判定します.

#include <iostream>
#include <array>

using namespace std;

class point {
public:
    double x, y;
    double operator%(const point &p) const;
    point operator-(const point &p) const;
};

// cross product
double point::operator%(const point &p) const {
    return this->x * p.y - this->y * p.x;
}

point point::operator-(const point &p) const {
    point r;
    r.x = this->x - p.x;
    r.y = this->y - p.y;
    return r;
}

int main() {
    array<point, 4> p;
    while (cin >> p[0].x >> p[0].y >> p[1].x >> p[1].y >> p[2].x >> p[2].y >> p[3].x >> p[3].y) {
        double c0 = (p[2] - p[0]) % (p[0] - p[3]);
        double c1 = (p[0] - p[1]) % (p[1] - p[3]);
        double c2 = (p[1] - p[2]) % (p[2] - p[3]);

        if ((c0 > 0 && c1 > 0 && c2 > 0) || (c0 < 0 && c1 < 0 && c2 < 0)) {
            cout << "YES" << endl;
        } else {
            cout << "NO" << endl;
        }
    }

    return 0;
}