round 1: update 100 unix kernel machines in the moon;
round 2: robot, turnright(int k), turnleft(int k), move(), and clean(), 设计一个系统把房间扫干净；
round 3: recommendation system for youtube videos.
round 4: stock 1 find maximum profit, and all the buying and selling point; Question 2: LRU
round 5: design the middle layer API, multiple thread, scalablity.
