I encounter a problem when I try to use a list as a 2d array.
li = [[0]*3]*3
print li
li[0][0] = 1
print li
then the weird thing happens the output would be:
[[0,0,0],[0,0,0],[0,0,0]]
[[1,0,0],[1,0,0],[1,0,0]]
why it's not
[[1,0,0],[0,0,0],[0,0,0]]
