for(int k = 1; k <= n; ++ k) {
        for(int i = 1; i <= n; ++ i) {
            if(i == k) continue;
            for(int j = 1; j <= n; ++ j) {
                if(i == j) continue;
                dn[i][j] = max(dn[i][j], dn[i][k] + dn[k][j]);
                dx[i][j] = min(dx[i][j], dx[i][k] + dx[k][j]);
            }
        }
    }

所以这题为啥要先枚举k呢？