SAP智力题：The number of balls ★★★★

There are N balls on a flat surface, you can watch from side, you can choose any angle, what you can see is shadows of the balls on the side. How many times at least do you need to watch so you are are sure to figure out the N?
There are N balls in 3D space, you can watch at any angle, what you can see is shadows of the balls on the side where you watch from. How many times at least do you need to watch so you are sure to figure out the N?
( https://www.sdn.sap.com/irj/scn/forum?forumID=410 )

所有跟帖： 

• I don't understand what is the shaows? Where is the light ? -jinjing- ♀ (0 bytes) () 10/27/2010 postreply 10:25:40

• 我理解可能是平行投影的意思吧(?), 光源在视线方向上的无穷远处... -与数学无关- ♂ (0 bytes) () 10/27/2010 postreply 12:46:29

• Thanks. -jinjing- ♀ (0 bytes) () 10/27/2010 postreply 14:50:21

• 3 times, independent of N. -好象- ♂ (0 bytes) () 10/27/2010 postreply 18:31:30