/in/foo.cc:13:5: error: 'int index' redeclared as different kind of symbol
 int index;
     ^~~~~
In file included from /usr/include/c++/6/cstring:42:0,
                 from /in/foo.cc:5:
/usr/include/string.h:477:1: note: previous declaration 'const char* index(const char*, int)'
 index (const char *__s, int __c) __THROW
 ^~~~~
/in/foo.cc: In function 'void tarjan(int)':
/in/foo.cc:17:21: error: no pre-increment operator for type
     DFN[x]=LOW[x]=++index;
                     ^~~~~
/in/foo.cc:23:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<v[x].size();i++)
                 ~^~~~~~~~~~~~