answer.code:25:5: error: ‘os’ does not name a type
   25 |     os st[N];
      |     ^~
answer.code: In member function ‘void optimize_with_pst::reset()’:
answer.code:27:29: error: ‘st’ was not declared in this scope; did you mean ‘std’?
   27 |         for(int a=0;a<N;++a)st[a].clear();
      |                             ^~
      |                             std
answer.code: In member function ‘void optimize_with_pst::iupd(int, int)’:
answer.code:30:12: error: ‘st’ was not declared in this scope; did you mean ‘std’?
   30 |         if(st[nd].find(i)!=st[nd].end())st[nd].erase(i);
      |            ^~
      |            std
answer.code: In member function ‘void optimize_with_pst::merge(int, int)’:
answer.code:34:12: error: ‘st’ was not declared in this scope; did you mean ‘std’?
   34 |         if(st[node1].size()<st[node2].size())st[node1].swap(st[node2]);
      |            ^~
      |            std
answer.code:35:20: error: ‘st’ was not declared in this scope; did you mean ‘std’?
   35 |         for(auto a:st[node2]){
      |                    ^~
      |                    std
answer.code: In member function ‘bool optimize_with_pst::pq(int, int)’:
answer.code:44:16: error: ‘st’ was not declared in this scope; did you mean ‘std’?
   44 |         return st[node].order_of_key(i+1);
      |                ^~
      |                std
answer.code: In function ‘int main()’:
answer.code:79:20: warning: ignoring return value of ‘size_t fread(void*, size_t, size_t, FILE*)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
   79 | signed main(){fread(Buf,1,BUFSIZE,stdin);
      |               ~~~~~^~~~~~~~~~~~~~~~~~~~~