tzoj3348 线段相交Ⅲ

 就是个解方程。

#include <bits/stdc++.h>
#define IO ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
using namespace std;

double ansx, ansy;

bool pingxing(double x1, double y1, double x2, double y2, double x3, double y3, double x4, double y4)
{
    return (y2 - y1) * (x4 - x3) == (x2 - x1) * (y4 - y3);
}

bool xiangjiao(double x1, double y1, double x2, double y2, double x3, double y3, double x4, double y4)
{
    double t1, t2;
    t1 = ((x3 - x1) * (y4 - y3) - (y3 - y1) * (x4 - x3)) / ((x2 - x1) * (y4 - y3) - (x4 - x3) * (y2 - y1));
    t2 = ((x1 - x3) * (y2 - y1) + (y3 - y1) * (x2 - x1)) / ((x4 - x3) * (y2 - y1) - (x2 - x1) * (y4 - y3));
    if(t1 >= 0 && t1 <= 1 && t2 >= 0 && t2 <= 1)
    {
        ansx = x1 + t1 * (x2 - x1);
        ansy = y1 + t1 * (y2 - y1);
        return 1;
    }
    return 0;
}

bool gongxian(double x1, double y1, double x2, double y2, double x3, double y3)
{
    return (y3 - y1) * (x2 - x1) == (x3 - x1) * (y2 - y1);
}

bool inside(double x1, double y1, double x2, double y2, double x3, double y3)
{
    return (x3 >= min(x1, x2) && x3 <= max(x1, x2) && y3 >= min(y1, y2) && y3 <= max(y1, y2));
}

signed main()
{
    IO;
    int t;
    cin >> t;
    double x1,  y1,  x2,  y2, x3,  y3,  x4,  y4;
    while(t--)
    {
        cin >> x1 >> y1 >> x2 >> y2 >> x3 >> y3 >> x4 >> y4;
        if (pingxing(x1, y1, x2, y2, x3, y3, x4, y4))
        {
            if (gongxian(x1, y1, x2, y2, x3, y3))
            {
                if (inside(x1, y1, x2, y2, x3, y3))
                {
                    cout << "YES" << endl;
                    continue;
                }

                if (inside(x1, y1, x2, y2, x4, y4))
                {
                    cout << "YES" << endl;
                    continue;
                }

            }
            cout << "NO" << endl;
            continue;
        }
        else
        {
            if (xiangjiao(x1, y1, x2, y2, x3, y3, x4, y4))
            {
                cout << "YES";
                if ((ansx != x1 || ansy != y1) && (ansx != x2 || ansy != y2) && (ansx != x3 || ansy != y3) && (ansx != x4 || ansy != y4))
                    cout << fixed << setprecision(3) << " (" << ansx << "," << ansy << ")";
                cout << endl;
            }
            else
                cout << "NO" << endl;
        }
    }
    return 0;
}