foo.cc: In function 'int dfs(int)':
foo.cc:43:32: error: no matching function for call to 'max(int)'
   dp[x]=max(dfs(e[i].to)+e[i].v);
                                ^
In file included from /usr/include/c++/7/bits/char_traits.h:39:0,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:2:
/usr/include/c++/7/bits/stl_algobase.h:219:5: note: candidate: template<class _Tp> const _Tp& std::max(const _Tp&, const _Tp&)
     max(const _Tp& __a, const _Tp& __b)
     ^~~
/usr/include/c++/7/bits/stl_algobase.h:219:5: note:   template argument deduction/substitution failed:
foo.cc:43:32: note:   candidate expects 2 arguments, 1 provided
   dp[x]=max(dfs(e[i].to)+e[i].v);
                                ^
In file included from /usr/include/c++/7/bits/char_traits.h:39:0,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from foo.cc:2:
/usr/include/c++/7/bits/stl_algobase.h:265:5: note: candidate: template<class _Tp, class _Compare> const _Tp& std::max(const _Tp&, const _Tp&, _Compare)
     max(const _Tp& __a, const _Tp& __b, _Compare __comp)
     ^~~
/usr/include/c++/7/bits/stl_algobase.h:265:5: note:   template argument deduction/substitution failed:
foo.cc:43:32: note:   candidate expects 3 arguments, 1 provided
   dp[x]=max(dfs(e[i].to)+e[i].v);
                                ^
In file included from /usr/include/c++/7/algorithm:62:0,
                 from foo.cc:3:
/usr/include/c++/7/bits/stl_algo.h:3462:5: note: candidate: template<class _Tp> _Tp std::max(std::initializer_list<_Tp>)
     max(initializer_list<_Tp> __l)
     ^~~
/usr/include/c++/7/bits/stl_algo.h:3462:5: note:   template argument deduction/substitution failed:
foo.cc:43:32: note:   mismatched types 'std::initializer_list<_Tp>' and 'int'
   dp[x]=max(dfs(e[i].to)+e[i].v);
                                ^
In file included from /usr/include/c++/7/algorithm:62:0,
                 from foo.cc:3:
/usr/include/c++/7/bits/stl_algo.h:3468:5: note: candidate: template<class _Tp, class _Compare> _Tp std::max(std::initializer_list<_Tp>, _Compare)
     max(initializer_list<_Tp> __l, _Compare __comp)
     ^~~
/usr/include/c++/7/bits/stl_algo.h:3468:5: note:   template argument deduction/substitution failed:
foo.cc:43:32: note:   mismatched types 'std::initializer_list<_Tp>' and 'int'
   dp[x]=max(dfs(e[i].to)+e[i].v);
                                ^

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。