广搜 BFS()

 1 void BFS(???){
 2     queue<node>q;
 3     node start, next;
 4     start = ???;
 5     q.push(start);
 6     
 7     while(!q.empty()){
 8         start = q.front();
 9         q.pop();
10         
11         //一系列迷幻操作
12             next = ???;
13             if(???) 
14                 q.push(next); 
15     }
16 }