0311. Sparse Matrix Multiplication

https://leetcode.com/problems/sparse-matrix-multiplication

Description

Given two sparse matrices mat1 of size m x k and mat2 of size k x n, return the result of mat1 x mat2. You may assume that multiplication is always possible.

Example 1:

**Input:** mat1 = [[1,0,0],[-1,0,3]], mat2 = [[7,0,0],[0,0,0],[0,0,1]]
**Output:** [[7,0,0],[-7,0,3]]

Example 2:

**Input:** mat1 = [[0]], mat2 = [[0]]
**Output:** [[0]]

Constraints:

• m == mat1.length

• k == mat1[i].length == mat2.length

• n == mat2[i].length

• 1 <= m, n, k <= 100

• -100 <= mat1[i][j], mat2[i][j] <= 100

ac

math problem.

class Solution {
    public int[][] multiply(int[][] A, int[][] B) {
        // edge cases
        if (A.length == 0 || A[0].length == 0 || B.length == 0 || B[0].length == 0)
            return new int[0][0];

        int rowa = A.length, cola = A[0].length, rowb = B.length, colb = B[0].length;
        int[][] res = new int[rowa][colb];

        for (int ra = 0; ra < rowa; ra++) {
            for (int ca = 0; ca < cola; ca++) {
                if (A[ra][ca] == 0) continue; // skip 0
                for (int cb = 0; cb < colb; cb++) {
                    res[ra][cb] += A[ra][ca] * B[ca][cb];
                }
            }
        }

        return res;
    }
}

/*
know how to do the math
*/