Language: C++

#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<ll,ll>;
#define rep(i,n) for(ll i=0;i<(ll)n;++i)

struct UnionFind{
  vector<int> d; //子なら親のID,根なら-サイズを保存
  UnionFind(int n=0) : d(n,-1) {} //最初はみんな根でサイズ1 (コンストラクタ :と{}の間には初期化子が入る)
  int find(int x){
    //自分が根なら自分を、そうでなければ親のfind結果を返す(再帰)
    if( d.at(x) < 0 )  return x;
    return d.at(x) = find(d.at(x)); //親のfindの結果で上書き(メモ化で木のサイズを小さくする)
  }
  bool unite(int x,int y){
    x = find(x); y = find(y);
    if( x == y ) return false; //元々同じグループなら、やることはない (falseはminimun spaning treeで使えるので便利)
    if( -d.at(x) < -d.at(y) ) swap(x,y); //サイズが大きい方に小さい方をくっつけたい
    d.at(x) += d.at(y); //xにyをくっつける
    d.at(y) = x;
    return true;
  }
  //同じ集合に属しているか判定する関数
  bool same(int x,int y){ return find(x) == find(y); }
  //要素の属する集合のサイズを返す関数
  int size(int x){ return -d.at( find(x) ); }
};

int main(){
  ll n,m;cin >> n >> m;
  vector<int> deg(n);
  vector<<vector<ll>> to(n,vector<ll>(0));
  UnionFind uf(n);
  //友達関係
  rep(i,m){
    int a,b;
    cin >> a >> b;
    a--; b--;
    deg.at(a)++;
    deg.at(b)++;
    uf.unite(a,b);
  }
  ll ans = -1;
  rep(i,n){
    ans = max(uf.size(i),ans);
  }
  cout << ans << endl;
  return 0;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:32:9: error: expected unqualified-id before '<<' token
   32 |   vector<<vector<ll>> to(n,vector<ll>(0));
      |         ^~
a.cc:45:14: error: no matching function for call to 'max(int, ll&)'
   45 |     ans = max(uf.size(i),ans);
      |           ~~~^~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_algobase.h:257:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::max(const _Tp&, const _Tp&)'
  257 |     max(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:257:5: note:   template argument deduction/substitution failed:
a.cc:45:14: note:   deduced conflicting types for parameter 'const _Tp' ('int' and 'll' {aka 'long long int'})
   45 |     ans = max(uf.size(i),ans);
      |           ~~~^~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::max(const _Tp&, const _Tp&, _Compare)'
  303 |     max(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note:   candidate expects 3 arguments, 2 provided
In file included from /usr/include/c++/14/algorithm:61:
/usr/include/c++/14/bits/stl_algo.h:5706:5: note: candidate: 'template<class _Tp> constexpr _Tp std::max(initializer_list<_Tp>)'
 5706 |     max(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5706:5: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5716:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::max(initializer_list<_Tp>, _Compare)'
 5716 |     max(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5716:5: note:   template argument deduction/substitution failed:
a.cc:45:14: note:   mismatched types 'std::initializer_list<_Tp>' and 'int'
   45 |     ans = max(uf.size(i),ans);
      |           ~~~^~~~~~~~~~~~~~~~