矩阵置零的程序 python小白有点不知道为啥这样不行(

class Solution:
def setZeroes(self, matrix: List[List[int]]) -> None:
"""
Do not return anything, modify matrix in-place instead.
"""
mark=[]
for i in range(len(matrix)):
for j in range(len(matrix[0])):
if matrix[i][j]==0:

mark.append((i,j))

for i,j in mark:
matrix[i]=[0]*len(matrix[0])

# matrix[:][j]=0 为啥这样就没修改呢
for l in range(len(matrix)):
matrix[l][j]=0