/in/foo.cc: In function 'bool BFS()':
/in/foo.cc:18:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         if (now.pos==a.size()-1){//到终点了
             ~~~~~~~^~~~~~~~~~~~
/in/foo.cc:36:25: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (next.pos<a.size()&&a[next.pos]!=' '){
                 ~~~~~~~~^~~~~~~~~