Solution.java

class Solution {
    public int[][] multiply(int[][] mat1, int[][] mat2) {
        int m = mat1.length, n = mat2[0].length;
        int[][] ans = new int[m][n];
        var g1 = f(mat1);
        var g2 = f(mat2);
        for (int i = 0; i < m; ++i) {
            for (int[] p : g1[i]) {
                int k = p[0], x = p[1];
                for (int[] q : g2[k]) {
                    int j = q[0], y = q[1];
                    ans[i][j] += x * y;
                }
            }
        }
        return ans;
    }

    private List<int[]>[] f(int[][] mat) {
        int m = mat.length, n = mat[0].length;
        List<int[]>[] g = new List[m];
        Arrays.setAll(g, i -> new ArrayList<>());
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (mat[i][j] != 0) {
                    g[i].add(new int[] {j, mat[i][j]});
                }
            }
        }
        return g;
    }
}