311. 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 m = A.length, n = A[0].length, nB = B[0].length;
        int[][] C = new int[m][nB];

        for (int row = 0; row < m; row++) {
            for (int col = 0; col < n; col++) {
                if (A[row][col] != 0) {
                    for (int j = 0; j < nB; j++) {
                        if (B[col][j] != 0) {
                            C[row][j] += A[row][col] * B[col][j];
                        }
                    }
                }
            }
        }
        return C;
    }
}

 

public class Solution {
    public int[][] multiply(int[][] A, int[][] B) {
        if (A == null || A[0] == null || B == null || B[0] == null) return null;
        int rowA = A.length, colA = A[0].length, colB = B[0].length;
        int[][] C = new int[rowA][colB];
        
        //Map<row, Map<col, val>>
        Map<Integer, Map<Integer, Integer>> tableB = new HashMap<>();

        // For each row in matrix B, if there is a non-zero value, put it in the hashMap.
        for (int row = 0; row < colA; row++) {
            tableB.put(row, new HashMap<Integer, Integer>());
            for (int col = 0; col < colB; col++) {
                if (B[row][col] != 0) {
                    tableB.get(row).put(col, B[row][col]);
                }
            }
        }

        for (int row = 0; row < rowA; row++) {
            for (int col = 0; col < colA; col++) {
                if (A[row][col] != 0) {
                    for (Integer j : tableB.get(col).keySet()) {
                        C[row][j] += A[row][col] * tableB.get(col).get(j);
                    }
                }
            }
        }
        return C;
    }
}