https://cses.fi/paste/408cea2327e7ddb2130854/
#include <bits/stdc++.h>#define mod 1000000007;#define ll long longusing namespace std;int main(){ll n;cin >> n;ll arr[n];for (int i = 0; i < n; ++i){cin >> arr[i];}ll ans = 0;ll start = 0, end = 0;unordered_set<ll> set;while (end < n ){if (set.find(arr[end]) == set.end()){set.insert(arr[end]);ans = max(ans, end - start + 1);end++;}else{set.erase(arr[start]);start++;}}cout << ans;return 0;}