Leetcode 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 |

Difficulty: Medium

Solution: Firstly, do the if (A[i][k] != 0)

public class Solution {
public int[][] multiply(int[][] A, int[][] B) {
int m = A.length, n = A[0].length, nB = B[0].length;
int[][] C = new int[m][nB];

for(int i = 0; i < m; i++) {
for(int k = 0; k < n; 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;
}
}