假设一个城市有n个小区，要实现n个小区之间的电网都能够相互接通，构造这个城市n个小区之间的电网，使总工程造价最低。请设计一个能够满足要求的造价方案。(Assuming that a city has n districts, in order to realize the interconnection of power grids between n districts, the power grids between N districts in this city are construc