foo.cc:5:13: error: array bound is not an integer constant before ']' token
 int pre[MAXN];
             ^
foo.cc: In function 'int find(int)':
foo.cc:8:11: error: 'pre' was not declared in this scope
  if (x == pre[x]) return x;
           ^~~
foo.cc:8:11: note: suggested alternative: 'drem'
  if (x == pre[x]) return x;
           ^~~
           drem
foo.cc:9:9: error: 'pre' was not declared in this scope
  return pre[x] = find(pre[x])
         ^~~
foo.cc:9:9: note: suggested alternative: 'drem'
  return pre[x] = find(pre[x])
         ^~~
         drem
foo.cc:10:1: error: expected ';' before '}' token
 }
 ^
foo.cc: At global scope:
foo.cc:12:11: error: expected identifier before '(' token
 void union(int x, int y){
           ^
foo.cc:12:12: error: expected unqualified-id before 'int'
 void union(int x, int y){
            ^~~
foo.cc:12:12: error: expected ')' before 'int'
foo.cc: In function 'int main()':
foo.cc:23:27: error: 'pre' was not declared in this scope
   for (int i=0; i<n; ++i) pre[i] = i;
                           ^~~
foo.cc:23:27: note: suggested alternative: 'drem'
   for (int i=0; i<n; ++i) pre[i] = i;
                           ^~~
                           drem
foo.cc:26:4: error: expected primary-expression before 'union'
    union(a,b);
    ^~~~~
foo.cc:25:9: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
    scanf("%d%d",&a,&b);
    ~~~~~^~~~~~~~~~~~~~
foo.cc:29:9: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
    scanf("%d%d",&a,&b);
    ~~~~~^~~~~~~~~~~~~~

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