/in/foo.cc:12:5: error: 'int index' redeclared as different kind of symbol
 int index, root;
     ^~~~~
In file included from /in/foo.cc:3:0:
/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:16:22: error: no pre-increment operator for type
  DFN[u] = Low[u] = ++index;
                      ^~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:50:24: error: overloaded function with no contextual type information
   ans = root = index = 0;
                        ^