如何向下移动嵌套 python 列表中的所有元素？

Question

we have a 4x4 board containing empty values, and 1s

board = [
[" "," ","1"," "],
[" "," ","1"," "],
["1","1"," "," "],
["1"," "," ","1"]
]

now we want to the final result to look like this?

gravity(board) == [
[" "," "," "," "],
[" "," "," "," "],
["1"," ","1"," "],
["1","1","1","1"]
]

Answer 1

Idea: For each column, you run from the bottom to top. If you found an "1", then assign it to the bottom.

for j in range(len(board[0])):
    cnt = len(board)-1
    for i in range(len(board)-1, -1, -1):
        if (board[i][j]=='1'):
            board[i][j]=''
            board[cnt][j]='1'
            cnt -= 1

Result:

[[' ' ' ' '' ' ']
 [' ' ' ' '' ' ']
 ['1' '' '1' ' ']
 ['1' '1' '1' '1']]