/in/foo.cc: In function 'bool BFS()':
/in/foo.cc:20:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         if (now.x==a.length()-1){
             ~~~~~^~~~~~~~~~~~~~
/in/foo.cc:37:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             if (next.x<a.size()&&a[next.x]!=' '){
                 ~~~~~~^~~~~~~~~