/in/foo.cc: In function 'int main()':
/in/foo.cc:21:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int i=0;i<a.length();i++) c=c*10+a[i]-'0';   //取出题目给的句子数量 
                     ~^~~~~~~~~~~
/in/foo.cc:22:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int i=4;i<b.length()-1;i++) d=d*10+b[i]-'0';  //取出题目给的时间复杂度 O(1)不影响 
                     ~^~~~~~~~~~~~~