The question and solution are as follows:
圖片參考:http://i707.photobucket.com/albums/ww74/stevieg90/ScreenHunter_04Jan161527.gif
(http://i707.photobucket.com/albums/ww74/stevieg90/ScreenHunter_04Jan161527.gif)
I don't know why it's 5*4*3*1 in case 1 and 4*4*3*2 in case 2? Please explain. Also, do you have any alternative way to solve it? thx!