foo.cc: In function 'int main()':
foo.cc:12:12: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(i=0; i<a.length(); i++){
           ~^~~~~~~~~~~
foo.cc:15:12: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(i=0; i<b.length(); i++){
           ~^~~~~~~~~~~
foo.cc:26:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   while(n != string::npos){   //从第一次查找到的位置开始往后继续查找（包括第一次找到的，如果找到了，进入循环，如果没找到则退出循环）
         ~~^~~~~~~~~

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