Language: C++

//Optional FAST

//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,fma,abm,mmx,avx,avx2,tune=native")

//Required Libraries

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/detail/standard_policies.hpp>

//Required namespaces

using namespace std;
using namespace __gnu_pbds;

//Required defines

#define endl '\n'

#define READ(X) cin>>X;
#define READV(X) long long X; cin>>X;
#define READAR(A,N) long long A[N]; for(long long i=0;i<N;i++) {cin>>A[i];}
#define rz(A,N) A.resize(N);
#define sz(X) (long long)(X.size())
#define pb push_back
#define pf push_front
#define fi first
#define se second
#define FORI(a,b,c) for(long long a=b;a<c;a++)
#define FORD(a,b,c) for(long long a=b;a>c;a--)

//Required typedefs

typedef tree<long long,null_type,less<long long>,rb_tree_tag,tree_order_statistics_node_update> ordered_set;
typedef tree<long long,null_type,greater<long long>,rb_tree_tag,tree_order_statistics_node_update> ordered_set1;
typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;
typedef pair<long long,long long> pll;

//Required Constants

const long long inf=(long long)1e18;
const long long MOD=(long long)(1e9+7);
const long long INIT=(long long)(1e6+1);
const long double PI=3.14159265358979;

// Required random number generators

// mt19937 gen_rand_int(chrono::steady_clock::now().time_since_epoch().count());
// mt19937_64 gen_rand_ll(chrono::steady_clock::now().time_since_epoch().count());

//Required Functions

ll power(ll x,ll y) 
{ 
    if (y == 0) 
        return 1; 
    ll p = power(x, y/2) % MOD; 
    p = (p * p) % MOD; 
  
    return (y%2 == 0)? p : (x * p) % MOD; 
}
ll modInverse(ll a) 
{ 
   return power(a,MOD-2); 
   
}

//Work

ll root[500001];
ll size[500001];
void initialise()
{
    for (ll i = 0; i < 500001; ++i)
    {
        root[i]=i;
    }
    for (ll i = 0; i < 500001; ++i)
    {
        size[i]=1;
    }
}
ll find_root(ll v)
{
    if(root[v]==v)
    {
        return v;
    }
    else
    {
        root[v]=find_root(root[v]);
        return root[v];
    }
}
bool find(ll a,ll b)
{
    return find_root(a)==find_root(b);
}
void _union(ll a,ll b)
{
    ll _rootA=find_root(a);
    ll _rootB=find_root(b);
    if(_rootA!=_rootB)
    {
        size[_rootA]+=size[_rootB];
        size[_rootB]=0;
        root[_rootB]=_rootA;
    }
}

int main()
{
    #ifndef ONLINE_JUDGE
    if (fopen("INPUT.txt", "r"))
    {
        freopen ("INPUT.txt" , "r" , stdin);
        //freopen ("OUTPUT.txt" , "w" , stdout);
    }
    #endif
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    clock_t clk;
    clk = clock();
//-----------------------------------------------------------------------------------------------------------//

    initialise();
    READV(N);
    READV(M);
    ll ans=0;
    FORI(i,0,M)
    {
        READV(x);
        READV(y);
        _union(x,y);
    }
    FORI(i,1,N+1)
    {
        ans=max(ans,size[i]);
    }
    cout<<ans<<endl;

//-----------------------------------------------------------------------------------------------------------//


    clk = clock() - clk;
    cerr << fixed << setprecision(6) << "Time: " << ((double)clk)/CLOCKS_PER_SEC << endl;

    return 0;
}

Compiler output:
a.cc: In function 'void initialise()':
a.cc:85:9: error: reference to 'size' is ambiguous
   85 |         size[i]=1;
      |         ^~~~
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:9:
/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:76:4: note:                 'll size [500001]'
   76 | ll size[500001];
      |    ^~~~
a.cc: In function 'void _union(ll, ll)':
a.cc:110:9: error: reference to 'size' is ambiguous
  110 |         size[_rootA]+=size[_rootB];
      |         ^~~~
/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:76:4: note:                 'll size [500001]'
   76 | ll size[500001];
      |    ^~~~
a.cc:110:23: error: reference to 'size' is ambiguous
  110 |         size[_rootA]+=size[_rootB];
      |                       ^~~~
/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:76:4: note:                 'll size [500001]'
   76 | ll size[500001];
      |    ^~~~
a.cc:111:9: error: reference to 'size' is ambiguous
  111 |         size[_rootB]=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:76:4: note:                 'll size [500001]'
   76 | ll size[500001];
      |    ^~~~
a.cc: In function 'int main()':
a.cc:145:21: error: reference to 'size' is ambiguous
  145 |         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:76:4: note:                 'll size [500001]'
   76 | ll size[500001];
      |    ^~~~