BFS八数码

BFS八数码问题：

#include <iostream>
#include <cstring>
#include <queue>
#include <unordered_map>
using namespace std;

int bfs(string start)
{
    string end = "12345678x";
    queue<string> q;
    unordered_map<string, int>d;
    q.push(start);
    d[start] = 0;
    int dx[4] = {-1,1,0,0},dy[4] = {0,0,1,-1};
    while(!q.empty())
    {
        string s = q.front();
        q.pop();
        int distance = d[s];
        if(s == end) return distance;
        int k = s.find('x');
        int x = k / 3, y = k % 3;
        for(int i = 0;i<4;i++)
        {
            int a = x + dx[i], b = y + dy[i];
            if(a >= 0 && a < 3 && b >= 0 && b < 3)
            {
                swap(s[k],s[a*3+b]);
                if(!d.count(s))
                {
                    d[s] = distance + 1;
                    q.push(s);
                }
                swap(s[k],s[a*3+b]);
            }
        }
        
    }
    
    return -1;
}
int main()
{
    string start;
    cin>>start;
    cout<<bfs(start)<<endl;
}