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