223. 矩形面积

思路（来源https://mp.weixin.qq.com/s/gypmkZFX87O1KzMt7XW0DA）：

class Solution {
public:

    int area(int x,int y)
    {
        return x*y;
    }
    int area(int x1,int y1,int x2,int y2)
    {
        return area(x2-x1,y2-y1);
    }
    int computeArea(int ax1, int ay1, int ax2, int ay2, int bx1, int by1, int bx2, int by2) {
        int cross_x=max(0,min(ax2,bx2)-max(ax1,bx1));
        int cross_y=max(0,min(ay2,by2)-max(ay1,by1));
        return area(ax1,ay1,ax2,ay2)+area(bx1,by1,bx2,by2)-area(cross_x,cross_y);
    }
};