311 Sparse Matrix Multiplication

Problem:

```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 |

```

Solutions:

``````public class Solution {
public int[][] multiply(int[][] A, int[][] B) {
int m = A.length, k = A[0].length, n = B[0].length;
int[][] result = new int[m][n];
for (int i = 0; i < m; i ++) {
for (int j = 0; j < n; j ++) {
int sum = 0;
for (int l = 0; l < k; l ++) {
sum +=A[i][l] * B[l][j];
}
result[i][j] = sum;
}
}
return result;
}
}
``````
``````public class Solution {
public int[][] multiply(int[][] A, int[][] B) {
int m = A.length, k = A[0].length, n = B[0].length;
int[][] result = new int[m][n];
for (int i = 0; i < m; i ++) {
for (int l = 0; l < k; l ++) {
if (A[i][l] != 0) {
for (int j = 0; j < n; j ++) {
result[i][j] +=A[i][l] * B[l][j];
}
}
}
}
return result;
}
}
``````