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 is only able to find one cross. After finding a possible cross, the function returns its x value, namely xCross.