311. Sparse Matrix Multiplication

Given two sparse matrices A and B , return the result of AB .

You may assume that A 's column number is equal to B 's row number.

Example:

Input:

A

 = [
  [ 1, 0, 0],
  [-1, 0, 3]
]
B

 = [
  [ 7, 0, 0 ],
  [ 0, 0, 0 ],
  [ 0, 0, 1 ]
]
Output:

     |  1 0 0 |   | 7 0 0 |   |  7 0 0 |
AB

 = | -1 0 3 | x | 0 0 0 | = | -7 0 3 |
                  | 0 0 1 |