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:

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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 |

感觉也没什么意思的题目

Ref: https://discuss.leetcode.com/topic/30625/easiest-java-solution

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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;
}