寻找平面上两条线的一个交点。两条直线以四个向量x1,y1,x2,y2的形式输入。横坐标向量x1,x2需单调递增。目前只找到一个交点即结束，函数返回值为交点的x坐标xCross。-Find a possible cross of two lines on a plan. The lines are in the form of four arrays, namely x1, y1, x2 and y2. x1 and x2 must be asscending. Now the function