foo.cc: In function 'void tarjan(int, int)':
foo.cc:20:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   20 |     for(int i=0;i<v[u].size();++i)
      |                 ~^~~~~~~~~~~~
foo.cc:27:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   27 |     for(int i=0;i<b[u].size();++i)
      |                 ~^~~~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:69:13: warning: unused variable 'pre' [-Wunused-variable]
   69 |     int x=1,pre;
      |             ^~~

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Found 6 testcases.
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