Olympiad Problem

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

const int N = 1e5 + 20;

int t[4 * N], added[4 * N];
vector<int> occ[N];

void inc(int v, int tl, int tr, int l, int r, int val){
  if (l > r) return;
  if (tl == l && tr == r){
    t[v] += val, added[v] += val;
    return;
  }
  int tm = (tl + tr) / 2;
  inc(2 * v + 1, tl, tm, l, min(r, tm), val);
  inc(2 * v + 2, tm + 1, tr, max(l, tm + 1), r, val);
  t[v] = max(t[2 * v + 1], t[2 * v + 2]) + added[v];
}

int main(){
  int n;
  cin >> n;
  int ans = 0;
  for (int i = 0; i < n; i++){
    int x;
    cin >> x;
    if (!occ[x].empty()){
      int l = (occ[x].size() == 1? -1 : occ[x].end()[-2]) + 1, r = occ[x].back();
      inc(0, 0, n - 1, l, r, +1);
      if (occ[x].size() > 1){
        r = l - 1, l = (occ[x].size() == 2? -1 : occ[x].end()[-3]) + 1;
        inc(0, 0, n - 1, l, r, -1);
      }
    }
    occ[x].push_back(i);
    ans = max(ans, t[0]);
  }
  cout << ans << endl;
}