foo.cc: In function 'int main()':
foo.cc:9:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<s.length();i++) if(s[i]=='5') v.push_back(i);
                 ~^~~~~~~~~~~
foo.cc:12:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=1;i<v.size();i++) ans-=(v[i]-v[i-1])*(v[i]-v[i-1]-1)/2; // 减去两个'5'之间的子串个数。 
                 ~^~~~~~~~~

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