Language: C++

#include <bits/stdc++.h>
//#include<fstream>
#include <ext/pb_ds/assoc_container.hpp>
using namespace std;
#define endl '\n'
#define pb push_back
#define fi first
#define se second
typedef long long ll;
#define double long double
#define pii pair<int,int>
#define pll pair<ll,ll>
#define fr(a,b,c) for(int a=b; a<=c; a++)
#define rep(a,b,c) for(int a=b; a<c; a++)
#define all(con) con.begin(),con.end()
#define w(x) int x; cin>>x; while(x--)
const ll infl=0x3f3f3f3f3f3f3f3fLL;
const int infi=0x3f3f3f3f;
//const int mod=998244353;
const int mod=1000000007;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<bool> vb;
#define pqb             priority_queue<int>
#define pqs             priority_queue<int,vi,greater<int> >
#define setbits(x)      __builtin_popcountll(x)
#define zrobits(x)      __builtin_ctzll(x)
void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}

template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifndef ONLINE_JUDGE
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif
//typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> pbds;

/*ofstream fout("bcount.out");
ifstream fin("bcount.in");*/
const int maxn = 2e5;
vector<ll>par(maxn+1);
vector<ll>size(maxn+1);
// void init()
// {
//     for(int i = 0;i<maxn+1;i++)
//     {
//         par[i] = i;
//         size[i] = 1;
//     }
// }

ll find(ll x)
{
    if(x == par[x])
        return x;
    return par[x] = find(par[x]);
}

void unite(ll x, ll y)
{
    x = find(x);
    y = find(y);
    if(x != y)
    {
        if(size[x] < size[y])
            swap(x,y);
        par[y] = x;
        size[x] += size[y];
        //size[y] = 0; Not sure about this though
    }

}

signed main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0),cout.tie(0);
    cout<<fixed<<setprecision(8);
    int n,m;
    cin>>n>>m;
    vi a(n);
    auto init = []()
    {
        for(int i = 0;i<maxn+1;i++)
        {
            par[i] = i;
            size[i] = 1;
        }
    };
    init();
    rep(i,0,m)
    {
        ll a,b;
        cin>>a>>b;
        unite(a,b);
    }
    ll ans = 0;
    rep(i,1,n+1)
     ans = max(ans,size[i]);
    cout<<ans<<endl;
    return 0;
}

Compiler output:
a.cc: In function 'void unite(ll, ll)':
a.cc:82:12: error: reference to 'size' is ambiguous
   82 |         if(size[x] < size[y])
      |            ^~~~
In file included from /usr/include/c++/14/string:53,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52,
                 from a.cc:1:
/usr/include/c++/14/bits/range_access.h:272:5: note: candidates are: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:                 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
a.cc:59:11: note:                 'std::vector<long long int> size'
   59 | vector<ll>size(maxn+1);
      |           ^~~~
a.cc:82:22: error: reference to 'size' is ambiguous
   82 |         if(size[x] < size[y])
      |                      ^~~~
/usr/include/c++/14/bits/range_access.h:272:5: note: candidates are: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:                 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
a.cc:59:11: note:                 'std::vector<long long int> size'
   59 | vector<ll>size(maxn+1);
      |           ^~~~
a.cc:85:9: error: reference to 'size' is ambiguous
   85 |         size[x] += size[y];
      |         ^~~~
/usr/include/c++/14/bits/range_access.h:272:5: note: candidates are: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:                 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
a.cc:59:11: note:                 'std::vector<long long int> size'
   59 | vector<ll>size(maxn+1);
      |           ^~~~
a.cc:85:20: error: reference to 'size' is ambiguous
   85 |         size[x] += size[y];
      |                    ^~~~
/usr/include/c++/14/bits/range_access.h:272:5: note: candidates are: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:                 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
a.cc:59:11: note:                 'std::vector<long long int> size'
   59 | vector<ll>size(maxn+1);
      |           ^~~~
a.cc: In lambda function:
a.cc:103:13: error: reference to 'size' is ambiguous
  103 |             size[i] = 1;
      |             ^~~~
/usr/include/c++/14/bits/range_access.h:272:5: note: candidates are: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:                 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
a.cc:59:11: note:                 'std::vector<long long int> size'
   59 | vector<ll>size(maxn+1);
      |           ^~~~
a.cc: In function 'int main()':
a.cc:115:20: error: reference to 'size' is ambiguous
  115 |      ans = max(ans,size[i]);
      |                    ^~~~
/usr/include/c++/14/bits/range_access.h:272:5: note: candidates are: 'template<class _Tp, long unsigned int _Nm> constexpr std::size_t std::size(const _Tp (&)[_Nm])'
  272 |     size(const _Tp (&)[_Nm]) noexcept
      |     ^~~~
/usr/include/c++/14/bits/range_access.h:262:5: note:                 'template<class _Container> constexpr decltype (__cont.size()) std::size(const _Container&)'
  262 |     size(const _Container& __cont) noexcept(noexcept(__cont.size()))
      |     ^~~~
a.cc:59:11: note:                 'std::vector<long long int> size'
   59 | vector<ll>size(maxn+1);
      |           ^~~~