/in/foo.cc: In function 'int main()':
/in/foo.cc:34:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j=0;j<a[i].size();j+=2)//之前写的j++ 查了半天 
               ~^~~~~~~~~~~~
/in/foo.cc:48:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int j=0;j<a[i].size();j+=2)
               ~^~~~~~~~~~~~