算法.积累20200411

1、ZC：求某个点 是否在 (凸)4边形 内部

　判断一个点是否在四边形内部_人工智能_San_Junipero的博客-CSDN博客.html（https://blog.csdn.net/San_Junipero/article/details/79172260）

　已知四边形的四个点，求一个点是否在四边形之内的解决方法_人工智能_laukaka的专栏-CSDN博客.html（https://blog.csdn.net/laukaka/article/details/45168439）

    private boolean isPointInRect(int x, int y)
    {
        final Point A = mLBPoint;
        final Point B = mLTPoint;
        final Point C = mRTPoint;
        final Point D = mRBPoint;
        final int a = (B.x - A.x)*(y - A.y) - (B.y - A.y)*(x - A.x);
        final int b = (C.x - B.x)*(y - B.y) - (C.y - B.y)*(x - B.x);
        final int c = (D.x - C.x)*(y - C.y) - (D.y - C.y)*(x - C.x);
        final int d = (A.x - D.x)*(y - D.y) - (A.y - D.y)*(x - D.x);
        if((a > 0 && b > 0 && c > 0 && d > 0) || (a < 0 && b < 0 && c < 0 && d < 0)) {
            return true;
        }
        
//        AB X AP = (b.x - a.x, b.y - a.y) x (p.x - a.x, p.y - a.y) = (b.x - a.x) * (p.y - a.y) - (b.y - a.y) * (p.x - a.x);
//        BC X BP = (c.x - b.x, c.y - b.y) x (p.x - b.x, p.y - b.y) = (c.x - b.x) * (p.y - b.y) - (c.y - b.y) * (p.x - b.x);
        return false; 
    }

 

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