#include <algorithm>
#include <cassert>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#include <cfloat>

using std::cerr;
using std::cin;
using std::cout;
using std::endl;

using namespace std;
typedef long long int64;

int64_t EuclidMod(int64_t v, int64_t m){
  if(0 <= v && v < m){
    return v;
  }else if(-m <= v && v < 0){
    return v + m;
  }else{
    int64_t result = v % m;
    if(result < 0){
      result += m;
    }
    return result;
  }
}

int64_t ModInv(int64_t a, int64_t m){
  int64_t b = m, u = 1, v = 0;
  while(b){
    int64_t t = a / b;
    a -= t * b;
    std::swap(a, b);
    u -= t * v;
    std::swap(u, v);
  }
  EuclidMod(u, m);
  return u;
}

// return base^exponent (MOD. mod).
int64_t RepeatedPowMod(int64_t base, int64_t exponent, int64_t mod) {
  if (exponent == 0)
    return 1;
  else if (exponent % 2 == 0) {
    int64_t root = RepeatedPowMod(base, exponent / 2, mod);
    return (root * root) % mod;
  } else {
    return (base * RepeatedPowMod(base, exponent - 1, mod)) % mod;
  }
}

template<int64_t mod>
class ModInt{
  int64_t value_;

public:
  explicit ModInt(const int64_t x = 0): value_(x % mod){
  };

  ModInt& operator=(const ModInt& another){
    value_=another.value_;
    return *this;
  }

  ModInt& operator^=(const ModInt& another){
    value_=RepeatedPowMod(value_,another.value_,mod);
    return *this;
  }

  ModInt& operator+=(const ModInt& another){
    value_ += another.value_;
    value_ = EuclidMod(value_, mod);
    return *this;
  }

  ModInt& operator-=(const ModInt& another){
    value_ -= another.value_;
    value_ = EuclidMod(value_, mod);
    return *this;
  }

  ModInt& operator*=(const ModInt& another){
    value_ *= another.value_;
    value_ = EuclidMod(value_, mod);
    return *this;
  }

  ModInt& operator/=(const ModInt& another){
    value_ = value_ * ModInv(another.value_, mod);
    value_ = EuclidMod(value_, mod);
    return *this;
  }

  ModInt& operator++(){
    value_ += 1;
    value_=EuclidMod(value_,mod);
    return *this;
  }

  ModInt operator++(int){
    ModInt tmp = *this;
    this->value_+=1;
    this->value_=EuclidMod(this->value_,mod);
    return tmp;
  }

  ModInt& operator--(){
    value_ -= 1;
    value_=EuclidMod(value_,mod);
    return *this;
  }

  ModInt operator--(int){
    ModInt tmp=*this;
    this->value_-=1;
    this->value_=EuclidMod(this->value_,mod);
    return tmp;
  }



  int64_t GetValue() const{
    return value_;
  }

};

template<int64_t mod>
ModInt<mod>
operator+(const ModInt<mod>& left, const ModInt<mod>& right){
  return ModInt<mod>(left) += right;
}

template<int64_t mod>
ModInt<mod>
operator-(const ModInt<mod>& left, const ModInt<mod>& right){
  return ModInt<mod>(left) -= right;
}

template<int64_t mod>
ModInt<mod>
operator/(const ModInt<mod>& left, const ModInt<mod>& right){
  return ModInt<mod>(left) /= right;
}

template<int64_t mod>
ModInt<mod>
operator*(const ModInt<mod>& left, const ModInt<mod>& right){
  return ModInt<mod>(left) *= right;
}

template<int64_t mod>
ModInt<mod>
operator^(const ModInt<mod>& left, const ModInt<mod>& right){
  return ModInt<mod>(left) ^= right;
}

template<int64_t mod>
std::ostream& operator<<(std::ostream& stream, const ModInt<mod>& mod_int){
  stream << mod_int.GetValue();
  return stream;
}

struct state{
    bool isupper;
    int pos;
    bool operator<(const state& another)const{
      return pos<another.pos;
    }
  };

int main(){
  cout << std::fixed << std::setprecision(16);
  cin.tie(nullptr);
  std::ios::sync_with_stdio(false);

  const int64_t MOD=1000000007;
  int n;
  cin>>n;

  ModInt<MOD> result(1);
  vector<int> pos[100001];
  for(int i=0;i<n;i++){
    int a;
    cin>>a;
    pos[a].push_back(i);
  }

  if(pos[0].size()>3||pos[0].size()<=0){
    cout<<0<<endl;
    return 0;
  }

  for(int i=1;i<n;i++){
    if(pos[i-1].size()<pos[i].size()){
      cout<<0<<endl;
      return 0;
    }
  }

  for(int i=0;i<n-1;i++){
    if(pos[i+1].size()==0){
      break;
    }
    vector<state> st;
    for(auto v:pos[i]){
      state pushed;
      pushed.isupper=false;
      pushed.pos=v;
      st.push_back(pushed);
    }
    for(auto v:pos[i+1]){
      state pushed;
      pushed.isupper=true;
      pushed.pos=v;
      st.push_back(pushed);
    }
    std::sort(st.begin(), st.end());
    int low_count=0;
    ModInt<MOD> tmp(1);
    for(auto s:st){
      if(!s.isupper){
        low_count++;
      }else{
        if(low_count<=0){
          cout<<0<<endl;
          return 0;
        }
        tmp*=ModInt<MOD>(low_count);
        low_count--;
      }
    }
    result*=tmp;

  }
  if(pos[0].size()>=2){
    result*=ModInt<MOD>(6);
  }else{
    result*=ModInt<MOD>(3);
  }

  cout<<result.GetValue()<<endl;





  return 0;
}

Measured memory_bytes: 5760