Solution2.cpp

class Solution {
public:
    vector<vector<int>> multiply(vector<vector<int>>& mat1, vector<vector<int>>& mat2) {
        int m = mat1.size(), n = mat2[0].size();
        vector<vector<int>> ans(m, vector<int>(n));
        auto g1 = f(mat1), g2 = f(mat2);
        for (int i = 0; i < m; ++i) {
            for (auto& [k, x] : g1[i]) {
                for (auto& [j, y] : g2[k]) {
                    ans[i][j] += x * y;
                }
            }
        }
        return ans;
    }

    vector<vector<pair<int, int>>> f(vector<vector<int>>& mat) {
        int m = mat.size(), n = mat[0].size();
        vector<vector<pair<int, int>>> g(m);
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (mat[i][j]) {
                    g[i].emplace_back(j, mat[i][j]);
                }
            }
        }
        return g;
    }
};