UVA 10250 - The Other Two Trees（几何）

#include <stdio.h>
#include <math.h>

double compute(double x1, double y1, double x2, double y2) {
    return (x1+x2+y1-y2)/2;
}

int main() {
    double x[4], y[4];

    while (scanf("%lf%lf%lf%lf", &x[0], &y[0], &x[1], &y[1]) != EOF) {
        if (fabs(x[0]-x[1])<1e-9 && fabs(y[0]-y[1])<1e-9) {
            printf("Impossible.\n");
            continue;
        }
        x[2] = compute(x[0], y[0], x[1], y[1]);
        x[3] = compute(x[1], y[1], x[0], y[0]);
        y[3] = compute(y[0], x[0], y[1], x[1]);
        y[2] = compute(y[1], x[1], y[0], x[0]);

        printf("%.10lf %.10lf %.10lf %.10lf\n", x[2], y[2], x[3], y[3]);
    }

    return 0;
}