Language: C++

#pragma optimize("unroll-loops,no-stack-protector")
#include <bits/stdc++.h>
using namespace std;
// using namespace __gnu_pbds;
// #include <ext/pb_ds/assoc_container.hpp>
// typedef tree<int,null_type,less<int>,rb_tree_tag,
// tree_order_statistics_node_update> indexed_set;
#define f(i,a,b) for(ll i=a;i<b;i++)
#define rep(i,n) f(i,0,n)
#define fd(i,a,b) for(ll i=a;i>=b;i--)
#define Max(a,b,c) max(a,max(b,c))
#define Min(a,b,c) min(a,min(b,c))
#define pb push_back
#define mp make_pair
#define vi vector< int >
#define pc(x)  __builtin_popcount (x)
#define vl vector< ll >
#define sorts(v) sort(v.begin(),v.end())
#define sortg(v) sort(v.begin(),v.end(),greater<ll>())
#define ss second
#define ff first
#define ll long long
#define ld long double
#define pii pair< int,int >
#define pll pair< ll,ll >
#define sz(a) a.size()
#define inf (1e18)
#define all(a) a.begin(),a.end()
#define tri pair<int,pii>
#define vii vector<pii>
#define vll vector<pll>
#define viii vector<tri>
#define mod 1000000007
#define mem(x,y) memset(x,y,sizeof(x))
#define pqueue priority_queue< int >
#define pdqueue priority_queue< int,vi ,greater< int > >
#define flush fflush(stdout) 
#define gcd __gcd
#define primeDEN 727999983
#define nx_pe next_permutation(s.begin(),s.end())
#define cl cout<<"\n"
#define sp(x) setprecision(x)
const ll N=2e5+5;
map<pll,ll>mm;
ll n,m;
vector<ll>v[N];
ll ans=0;
ll size=0;
bool vis[N];

void dfs(ll u)
{
    vis[u]=1;
    size++;
    for(ll x:v[u])
    {
        if(!vis[x])
        {
            dfs(x);
        }
    }
}

void solve()
{
   cin>>n>>m;
   rep(i,m)
   {
       ll x,y;
       cin>>x>>y;
       if(mm.find({x,y})==mm.end())
       {
           v[x].pb(y);
           v[y].pb(x);
           mm[{x,y}]++;
           mm[{y,x}]++;
       }
   }
   f(i,1,n+1)
   {
       if(!vis[i])
       {
           size=0;
           dfs(i);
           ans=max(ans,size);
       }
   }
   cout<<ans<<"\n";
}


    
int main()
{
    #ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin) ;
    freopen("ouput.txt", "w", stdout) ;
    #endif
    ios_base::sync_with_stdio(false);
    cin.tie(NULL) ; cout.tie(NULL) ;
    std::ios::sync_with_stdio(false);
    ll te=1;
    // cin>>te;
    while(te--){
        solve();
    }
    
}


Compiler output:
a.cc: In function 'void dfs(long long int)':
a.cc:54:5: error: reference to 'size' is ambiguous
   54 |     size++;
      |     ^~~~
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:2:
/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:48:4: note:                 'long long int size'
   48 | ll size=0;
      |    ^~~~
a.cc: In function 'void solve()':
a.cc:83:12: error: reference to 'size' is ambiguous
   83 |            size=0;
      |            ^~~~
/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:48:4: note:                 'long long int size'
   48 | ll size=0;
      |    ^~~~
a.cc:85:24: error: reference to 'size' is ambiguous
   85 |            ans=max(ans,size);
      |                        ^~~~
/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:48:4: note:                 'long long int size'
   48 | ll size=0;
      |    ^~~~