SparseMatrixMultiplication.java

package com.fishercoder.solutions;

/**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 SparseMatrixMultiplication {

    public int[][] multiply(int[][] A, int[][] B) {
        int m = A.length, n = A[0].length, p = B[0].length;
        int[][] C = new int[m][p];
        for(int i = 0; i < m; i++){
            for(int j = 0; j < n; j++){
                if(A[i][j] != 0){
                    for(int k = 0; k < p; k++){
                        if(B[j][k] != 0) C[i][k] += A[i][j]*B[j][k];
                    }
                }
            }
        }
        return C;
    }

}