一段代码：编写利用中点算法绘制两条直线，分别是：L1(Point(20, 10), Point(100, 180))， L2(Point(-50, 100), Point(100, -50))。

在许多编程语言中，你可以使用中点公式来计算两线段的交点，然后利用这个交点信息来绘制两条直线。首先，我们需要定义一个Point类来表示二维坐标。这里我将以Python为例展示如何做：

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

def calculate_midpoint(line1_start, line1_end, line2_start, line2_end):
    # 计算每条直线的中点
    midpoint_line1 = (line1_start.x + line1_end.x) / 2, (line1_start.y + line1_end.y) / 2
    midpoint_line2 = (line2_start.x + line2_end.x) / 2, (line2_start.y + line2_end.y) / 2

    # 检查是否平行，平行则无交点
    if line1_start.y - line1_end.y == line2_start.y - line2_end.y:
        return None

    # 使用两点式公式计算交点（如果需要考虑垂直线的情况）
    x_intersect = (midpoint_line2[1] - midpoint_line1[1]) * (line1_start.x - midpoint_line1[0]) / \
                  (line1_start.y - midpoint_line1[1] - (midpoint_line2[1] - midpoint_line1[1]))
    y_intersect = midpoint_line1[1] + (x_intersect - midpoint_line1[0]) * (line1_start.y - midpoint_line1[1]) / \
                  (line1_start.x - midpoint_line1[0])

    return Point(x_intersect, y_intersect)

# 定义两条直线
line1_start = Point(20, 10)
line1_end = Point(100, 180)
line2_start = Point(-50, 100)
line2_end = Point(100, -50)

intersection = calculate_midpoint(line1_start, line1_end, line2_start, line2_end)
if intersection is not None:
    print(f"两条直线的交点是: ({intersection.x}, {intersection.y})")
else:
    print("两条直线平行，没有交点.")

在这个例子中，我们首先检查了两条直线是否平行，如果平行则返回None，表示它们没有交点。然后通过交叉乘积法计算出交点。