explain the "70"

來源: longsky 於 2010-01-18 09:56:06 [] [舊帖] [給我悄悄話] 閱讀數 : (491 bytes)
本帖於 2010-04-08 17:50:51 時間, 由版主 於德利 編輯

first of all, think the simplest case, a square:

b a 1 0

d c 2 1

# of a->d = # of a->b + # of a->c
# of ab = 1, # of ac =1, therefore # of ad = 1+1=2

now you can mark the number of paths for each intersection points on your chart, which is :

1 1 1 1 0

5 4 3 2 1

15 10 6 3 1

35 20 10 4 1

70 35 15 5 1

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