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 |

public class Solution {
    public int[][] multiply(int[][] A, int[][] B) 
    {
        int ma = A.length;
        int na = A[0].length;
        int mb = B.length;   //na = mb
        int nb = B[0].length;
        
        int[][] C = new int[ma][nb];
        for(int i=0; i<ma;i++)
        {
            for(int k=0; k<na;k++)
            {
               if (A[i][k] != 0) 
               {
                    for (int j = 0; j < nb; j++) 
                    {
                        if (B[k][j] != 0) C[i][j] += A[i][k] * B[k][j];
                    }
                }
                
            }
            
        }
        return C;
    }
}

reference: https://leetcode.com/discuss/71912/easiest-java-solution