请使用C++语言编写：应用宽度优先搜索求解重排九宫问题

以下是使用C++语言编写的应用宽度优先搜索求解重排九宫问题的代码：

#include <iostream>
#include <queue>
#include <unordered_set>
using namespace std;

//定义九宫格状态
struct State {
    int nums[9]; //九宫格状态
    int zero_pos; //空格所在的位置
    string path; //记录路径
};

//判断九宫格状态是否已经达到目标状态
bool is_goal(State& s) {
    for(int i=0; i<8; i++) {
        if(s.nums[i] != i+1) {
            return false;
        }
    }
    return (s.nums[8] == 0);
}

//交换九宫格状态中的两个数
State swap_num(State& s, int pos1, int pos2) {
    State new_state = s;
    int temp = new_state.nums[pos1];
    new_state.nums[pos1] = new_state.nums[pos2];
    new_state.nums[pos2] = temp;
    return new_state;
}

//宽度优先搜索求解重排九宫问题
string bfs(State& start_state) {
    queue<State> q;
    unordered_set<string> visited; //用哈希表记录已经访问过的状态
    q.push(start_state);
    visited.insert(string(start_state.nums, start_state.nums+9)); //将初始状态加入已访问状态
    while(!q.empty()) {
        State cur_state = q.front();
        q.pop();
        //尝试移动空格
        if(cur_state.zero_pos >= 3) { //空格可以向上移动
            State new_state = swap_num(cur_state, cur_state.zero_pos, cur_state.zero_pos-3);
            if(visited.find(string(new_state.nums, new_state.nums+9)) == visited.end()) { //新状态未访问过
                new_state.zero_pos = cur_state.zero_pos-3;
                new_state.path = cur_state.path + "u";
                if(is_goal(new_state)) { //新状态为目标状态
                    return new_state.path;
                }
                q.push(new_state); //将新状态加入队列
                visited.insert(string(new_state.nums, new_state.nums+9)); //将新状态加入已访问状态
            }
        }
        if(cur_state.zero_pos <= 5) { //空格可以向下移动
            State new_state = swap_num(cur_state, cur_state.zero_pos, cur_state.zero_pos+3);
            if(visited.find(string(new_state.nums, new_state.nums+9)) == visited.end()) { //新状态未访问过
                new_state.zero_pos = cur_state.zero_pos+3;
                new_state.path = cur_state.path + "d";
                if(is_goal(new_state)) { //新状态为目标状态
                    return new_state.path;
                }
                q.push(new_state); //将新状态加入队列
                visited.insert(string(new_state.nums, new_state.nums+9)); //将新状态加入已访问状态
            }
        }
        if(cur_state.zero_pos % 3 != 0) { //空格可以向左移动
            State new_state = swap_num(cur_state, cur_state.zero_pos, cur_state.zero_pos-1);
            if(visited.find(string(new_state.nums, new_state.nums+9)) == visited.end()) { //新状态未访问过
                new_state.zero_pos = cur_state.zero_pos-1;
                new_state.path = cur_state.path + "l";
                if(is_goal(new_state)) { //新状态为目标状态
                    return new_state.path;
                }
                q.push(new_state); //将新状态加入队列
                visited.insert(string(new_state.nums, new_state.nums+9)); //将新状态加入已访问状态
            }
        }
        if(cur_state.zero_pos % 3 != 2) { //空格可以向右移动
            State new_state = swap_num(cur_state, cur_state.zero_pos, cur_state.zero_pos+1);
            if(visited.find(string(new_state.nums, new_state.nums+9)) == visited.end()) { //新状态未访问过
                new_state.zero_pos = cur_state.zero_pos+1;
                new_state.path = cur_state.path + "r";
                if(is_goal(new_state)) { //新状态为目标状态
                    return new_state.path;
                }
                q.push(new_state); //将新状态加入队列
                visited.insert(string(new_state.nums, new_state.nums+9)); //将新状态加入已访问状态
            }
        }
    }
    return ""; //无解
}

int main() {
    State start_state = {{2, 8, 3, 1, 0, 4, 7, 6, 5}, 4, ""}; //初始状态
    string path = bfs(start_state);
    if(path == "") { //无解
        cout << "No solution." << endl;
    } else {
        cout << "Solution: " << path << endl;
    }
    return 0;
}

该代码定义了一个九宫格状态的结构体 `State`，其中包含九宫格状态、空格所在位置和记录路径。使用队列和哈希表记录已经访问过的状态，在宽度优先搜索过程中，尝试向上、下、左、右四个方向移动空格，生成新的状态，并将新的状态加入队列和哈希表中。当新状态为目标状态时，返回路径。如果搜索完整个状态空间都没有找到解，则返回无解。