設以線段P1P2作為矩形R的對角線，以Q1Q2為矩形T的對角線，若矩形R和T不相交，則線段不會相交。

設P1 = (x1，y1), P2 = (x2, y2), Q1 = (x3, y3), Q2 = (x4, y4);

將矩形R的x座標的最小邊界用minRX = (x1, x2)表示，以此類推。

若兩矩形相交，設為矩形F。則矩形F的x座標的最小邊界minFX = max(minRX, minTX), minFY = max(minRY, minTY),

maxFX = min(maxRX, maxTX), maxFY = min(maxFY, maxTY).

得到F的各個值後，只要判斷矩形F是否成立就可以判斷是否相交了， 若minFX > maxFX, 或minFY > maxFY則矩形不相交，否則相交。

AC程式碼如下：

#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
double _max(double x, double y)
{
    if(x > y)
        return x;
    else
        return y;
}
double _min(double x, double y)
{
    if(x < y)
        return x;
    else
        return y;
}
int main()
{
    double x1, y1, x2, y2, x3, y3, x4, y4, area;
    while(scanf("%lf%lf%lf%lf%lf%lf%lf%lf", &x1, &y1, &x2, &y2, &x3, &y3, &x4, &y4) != EOF)
    {
        double minRX = min(x1, x2), minRY = min(y1, y2), maxRX = max(x1, x2), maxRY = max(y1, y2);
        double minTX = min(x3, x4), minTY = min(y3, y4), maxTX = max(x3, x4), maxTY = max(y3, y4);
        x1 = _max(minRX, minTX), x2 = _min(maxRX, maxTX);
        y1 = _max(minRY, minTY), y2 = _min(maxRY, maxTY);
        if(x1 > x2 || y1 > y2)
            area = 0.0;
        else
        {
            double a = x2 - x1, b = y2 - y1;
            area = a*b;
        }
        printf("%.2f\n", area);
    }

    return 0;
}