Language: C++

#include <bits/stdc++.h>
using namespace std;

#define pii  pair<int, int>
#define vi vector<int >
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define x first
#define y second
#define sz(x) (x).size()
#define all(x) (x).begin(), (x).end() //Forward traversal
#define rall(x) (x).rbegin, (x).rend() //reverse traversal

#define ll long long
#define ld long double

const ld EPS = 10e-9;
const int INF = 1e9 + 5;
const long long INFLL = (ll)INF * (ll)INF;
const ll MOD = 1e9 + 7;
const ld PI = 3.14159265358979323846;

template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v)
{
    os << "[";
    for (int i = 0; i < (int)v.size(); ++i) {
        os << v[i];
        if (i != (int)v.size() - 1)
            os << ", ";
    }
    os << "]\n";
    return os;
}

template <typename T>
ostream& operator<<(ostream& os, const set<T>& v)
{
    os << "{";
    for (const auto& it : v) {
        os << it;
        if (it != *v.rbegin())
            os << ", ";
    }
    os << "}\n";
    return os;
}

template <typename T>
ostream& operator<<(ostream& os, const unordered_set<T>& v)
{
    os << "{";
    for (const auto& it : v) {
        os << it;
        if (it != *v.rbegin())
            os << ", ";
    }
    os << "}\n";
    return os;
}

template <typename T, typename S>
ostream& operator<<(ostream& os, const map<T, S>& v)
{
    os << "{";
    for (const auto& it : v) {
        os << it.first << " : "
           << it.second;

        if( it != *v.rbegin())
            os  << ", ";
    }

    os << "}\n";
    return os;
}

template <typename T, typename S>
ostream& operator<<(ostream& os, const unordered_map<T, S>& v)
{
    os << "{";
    for (const auto& it : v) {
        os << it.first << " : "
           << it.second;

        if( it != *v.rbegin())
            os  << ", ";
    }

    os << "}\n";
    return os;
}

template <typename T, typename S>
ostream& operator<<(ostream& os, const pair<T, S>& v)
{
    os << "(";
    os << v.first << ", "
       << v.second << ")";
    os << "\n";
    return os;
}

#define DEBUG

#ifdef DEBUG
#define debug(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1>
void __f(const char* name, Arg1&& arg1){
    cerr << name << " : " << arg1;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args){
    const char* comma = strchr(names + 1, ',');cerr.write(names, comma - names) << " : " << arg1<<" | ";__f(comma+1, args...);
}
#else
#define debug(...)
#endif


// __gcd(x, y)

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);

    ll N, K;
    cin >> N >> K;

    vector<ll> a(N);
    for(int i = 0; i < N; i++)
        cin >> a[i];

//     ll l = a[0]-1;
//     ll r = a[a[0]-1]-1;
//     while(l!=r){
//         l = a[l]-1;
//         r = a[a[r]-1]-1;
//     }
//     l = 0;
//     while(l!=r){
//         l=a[l]-1;
//         r=a[r]-1;
//     }
//     //floyd warshall cycle detection algorithm ends here to find start of cycle

//     ll cycleStar = l;  //starting point of cycle
//     ll cnt = 0;  //number of elements before the starting point of cycle
//     l = 0;
//     while(l!=cycleStar){
//         cnt++;
//         l = a[l]-1;
//     }

//     ll pos = cnt;  //number of elements before the starting point of cycle
//     ll c = 1;   //length of cycle(num of elements in it)
//     l = cycleStar;
//     while(a[l]-1!=cycleStar){
//         l=a[l]-1;
//         c++;
//     }

    ll i = 0;
    unordered_set<ll> visited;
    visited.insert(0);
    ll pos2 = -1;
    ll c2 = 1;
    int lenBefore = 0;
    while(true)
    {
        if(visited.count(a[i]-1))
        {
            i = a[i]-1;
            pos2 = 0;
            while(pos2 != i)
            {
                pos2 = a[pos2]-1;
                lenBefore++;
            }
            pos2 = i;
            while(pos2 != a[i]-1)
            {
                c2++;
                i = a[i]-1;
            }
            break;
        }
        visited.insert(a[i]-1);
        i = a[i]-1;
    }

    if(pos != lenBefore)
    {
        throw runtime_error(to_string(pos) + " my: " + to_string(lenBefore));
    }

    ll ans = 0;
    while(lenBefore != 0 && K != 0)
    {
        ans = a[ans]-1;
        lenBefore--;
        K--;
    }
    if(K == 0)
    {
        cout << ans+1 << endl;
        return 0;
    }

    ll n = K % c2;
    while(n)
    {
        ans = a[ans]-1;
        n--;
    }

    cout << ans+1 << endl;
//    if(K <= pos)
//    {
//        ll ans = 0;
//        while(K)
//        {
//            K--;
//            ans = a[ans]-1;
//        }
//        cout << ans+1 << endl;
//    }
//    else {
//        ll ans = 0;
//        int pos_c = pos;
//        while(pos_c)
//        {
//            pos_c--;
//            ans = a[ans]-1;
//        }
//        int n = (K - pos) % c;
//        while(n)
//        {
//            n--;
//            ans = a[ans]-1;
//        }
//        cout << ans+1 << endl;
//    }
    return 0;
}



Compiler output:
a.cc: In function 'int main()':
a.cc:194:8: error: 'pos' was not declared in this scope; did you mean 'pos2'?
  194 |     if(pos != lenBefore)
      |        ^~~
      |        pos2