bfs通解，，，用了set存放狀態以保證第一次找到目標答案

 1 //
 2 // Created by snnnow on 2020/7/29.
 3 //
 4 
 5 #include<bits/stdc++.h>
 6 using namespace std;
 7 struct node {
 8     string str;
 9     int step;
10     int pos;
11     node (string str,int step,int pos):str(str),pos(pos),step(step){}
12 };
13 set <string> vis;
 
14 queue <node> q;
15 void Insert_Inq(node temp,int i)
16 {
17     string str = temp.str;
18     swap(str[temp.pos],str[(temp.pos+i+9)%9]);
19     if(vis.count(str)==0)
20     {
21         vis.insert(str);
22         node n (str,temp.step+1,(temp.pos+i+9)%9);
23         q.push(n);
24     }
25 }
 
26 int main ()
27 {
28 
29     node first ("012345678",0,0);
30     q.push(first);
31     while (!q.empty())
32     {
33         node temp = q.front();
34         if(temp.str=="087654321")
35         {
36             cout<<temp.step;
37             break;
38         }
39         else {
40             Insert_Inq(temp,1
 
);
41             Insert_Inq(temp,-1);
42             Insert_Inq(temp,2);
43             Insert_Inq(temp,-2);
44             q.pop();
45         }
46     }
47     return 0;
48 }