code: def multiply(self, a, b):
      
      if a is None or b is None: return None
      m, n, l = len(a), len(b[0]), len(b[0])
      if len(b) != n:
          raise Exception("A's column number must be equal to B's row number.")
      c = [[0 for _ in range(l)] for _ in range(m)]
      for i, row in enumerate(a):
          for k, eleA in enumerate(row):
              if eleA:
                  for j, eleB in enumerate(b[k]):
                      if eleB: c[i][j] += eleA * eleB
      return c
docs: :type A: List[List[int]]
      :type B: List[List[int]]
      :rtype: List[List[int]]