/in/foo.cc:11:5: error: 'int index' redeclared as different kind of symbol
 int index,ans;
     ^~~~~
In file included from /usr/include/c++/7/cstring:42:0,
                 from /in/foo.cc:3:
/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:29:10: error: no post-increment operator for type
     index++;
          ^~
/in/foo.cc:30:12: error: cannot resolve overloaded function 'index' based on conversion to type 'int'
     dfn[u]=index;
            ^~~~~
/in/foo.cc:31:12: error: cannot resolve overloaded function 'index' based on conversion to type 'int'
     low[u]=index;
            ^~~~~