/in/foo.cc:7:28: error: 'int index' redeclared as different kind of symbol
 int dfn[N],low[N],stack[N],index;
                            ^~~~~
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:35:21: error: no pre-increment operator for type
     dfn[x]=low[x]=++index;
                     ^~~~~