Use a vector to represent matrix A

```
class Solution(object):
def multiply(self, A, B):
"""
:type A: List[List[int]]
:type B: List[List[int]]
:rtype: List[List[int]]
"""
m, p = len(A), len(B[0])
Ai = [[i for i, v in enumerate(r) if v] if any(r) else [] for r in A]
res = [[0] * p for i in range(m)]
for i, r in enumerate(Ai):
if not r:
continue
for k in range(p):
for j, v in enumerate(r):
res[i][k] += A[i][v] * B[v][k]
return res
```