foo.cc: In function 'int main()':
foo.cc:11:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<s.size();i++){//移项变号
               ~^~~~~~~~~
foo.cc:18:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<s.size();i++){
               ~^~~~~~~~~
foo.cc:9:8: warning: 'c' may be used uninitialized in this function [-Wmaybe-uninitialized]
   char c;
        ^

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