311. Sparse Matrix Multiplication

def multiply(self, A, B):
        """
        :type A: List[List[int]]
        :type B: List[List[int]]
        :rtype: List[List[int]]
        """
        m, n, nB = len(A), len(A[0]), len(B[0])
        res = [[0 for _ in xrange(nB)] for _ in xrange(m)]
        for i in xrange(m):
            for j in xrange(n):
                if A[i][j]:
                    for k in xrange(nB):
                        res[i][k] += A[i][j] * B[j][k]
        return res