Language: C++

#include <bits/stdc++.h>
typedef long long ll;
const ll INF = 1e9;
const ll MOD = 10007; // 1e9+7
const ll LINF = 1e18;
using namespace std;

#define dump(x)  cout << #x << " = " << (x) << endl;
#define YES(n) cout << ((n) ? "YES" : "NO"  ) << endl
#define Yes(n) cout << ((n) ? "Yes" : "No"  ) << endl
#define POSSIBLE(n) cout << ((n) ? "POSSIBLE" : "IMPOSSIBLE"  ) << endl
#define Possible(n) cout << ((n) ? "Possible" : "Impossible"  ) << endl
#define possible(n) cout << ((n) ? "possible" : "impossible"  ) << endl

#define SANKOU(n,a,b) cout << ((n) ? (#a) : (#b) ) << endl

#define FOR(i,a,b) for(ll i=(a);i<(b);++i)
#define REP(i,n) for(ll i=0;i<(n);++i)
#define REPR(i,n) for(ll i=n;i>=0;i--)

#define FOREACH(x,a) for(auto&& (x) : (a) )

#define WFA(d,v) REP(k,v)REP(i,v)REP(j,v)d[i][j]=min(d[i][j],d[i][k]+d[k][j])

#define SCOUT(x) cout<<(x)<<" "
#define ENDL cout<<endl

#define VECCIN(x) for(auto&youso_: (x) )cin>>youso_
#define VECIN2(x,y) REP(i,x.size())cin>>x[i]>>y[i]
#define VECCOUT(x) if(1){for(auto tt=x.begin();tt!=x.end();tt++){if(tt!=x.begin())cout<<" ";cout<<(*tt);}cout<<endl;}

#define ALL(obj) (obj).begin(),(obj).end()

#define EXIST(n,x) (find(ALL(n),x)!=n.end())
#define UNIQUE(obj) sort(ALL( obj )); obj.erase(unique(ALL(obj)),obj.end())
#define EN(x) if(1){cout<<#x<<endl;return 0;}
#define COUT(x) cout<<(x)<<endl
void CINT(){}
template <class Head,class... Tail>
void CINT(Head&& head,Tail&&... tail){
    cin>>head;
    CINT(move(tail)...);
}
#define CIN(...) ll __VA_ARGS__;CINT(__VA_ARGS__)
#define LCIN(...) ll __VA_ARGS__;CINT(__VA_ARGS__)
#define SCIN(...) string __VA_ARGS__;CINT(__VA_ARGS__)

template <class T = ll>
T IN(){T x;cin>>x;return (x);}

template <class Head>
void VT(Head head){}
template <class Head,class Seco,class... Tail>
void VT(Head&& head,Seco&& seco,Tail&&... tail){
    seco.resize(head);
    VT(head,move(tail)...);
}
void VT2(){}
template <class Head,class... Tail>
void VT2(Head&& head,Tail&&... tail){
  VECCIN(head);
  VT2(move(tail)...);
}

template <class Head>
void VT3(Head&& head){}
template <class Head,class Seco,class... Tail>
void VT3(Head&& head,Seco&& seco,Tail&&... tail){
  seco[head]=IN();
  VT3(head,move(tail)...);
}

#define VC1(n,...) V __VA_ARGS__;VT(n,__VA_ARGS__);VT2(__VA_ARGS__); //aaabbbccc
#define VC2(n,...) V __VA_ARGS__;VT(n,__VA_ARGS__);REP(i,n)VT3(i,__VA_ARGS__); //abcabcabc

// #include <boost/multiprecision/cpp_ll.hpp>
// using namespace boost::multiprecision; // cpp_ll

#define P pair<ll,ll>
#define V vector<ll>
#define M map<ll,ll>
#define S set<ll>

#define pb(a) push_back(a)
#define mp make_pair

signed main(){
    CIN(n);
    VC1(n,a);
    vector<P> b(1, make_pair(a[0], 1));
    FOR(i,1,n-1){
        if(b.back().first == a[i])
            ++b.back().second;
        else
            b.emplace_back(a[i], 1);
    }

    auto f = [&](int p){
        unordered_map<int,int> m;
        int minu = 0;

        function<bool(int,int)> add = [&](int pos, int num){
            if(pos == -1)
                return false;
            m[pos] += num;
            int aft = m[pos];
            m[pos] %= p;
            if(aft != m[pos]){
                return add(pos - 1, aft / p);
            }
            return true;
        };

        FOREACH(x, b){
            if(minu < x.first){
                minu = x.first;
                --x.second;
            }
            else if(minu > x.first){
                minu = x.first;
                m.erase(m.upper_bound(minu), m.end());
            }
            if(x.second)
                if(!add(minu, x.second))
                    return false;
        }
        return true;
    };

    int st = 0; int en = n + 1;
    while(en - st > 1){
        int num = (st + en) / 2;
        (f(num) ? en : st) = num;
    }
    cout << en << endl;

}


Compiler output:
a.cc: In lambda function:
a.cc:121:27: error: 'class std::unordered_map<int, int>' has no member named 'upper_bound'
  121 |                 m.erase(m.upper_bound(minu), m.end());
      |                           ^~~~~~~~~~~