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:

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

B = [
  [ 7, 0, 0 ],
  [ 0, 0, 0 ],
  [ 0, 0, 1 ]
]


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

class Solution {
public:
    vector<vector<int>> multiply(vector<vector<int>>& A, vector<vector<int>>& B) {
        int m = A.size();
        int n = B[0].size();
         vector<vector<int>>res(m,vector<int>(n));
          for(int i = 0;i<A.size();i++)
               for(int k = 0;k<A[0].size();k++)
                   if(A[i][k])
                   {
                     for(int j = 0;j<B[0].size();j++)
                       if(B[k][j]) res[i][j] += A[i][k]*B[k][j];
                   }
        return res;
    }
};