#include <bits/stdc++.h>
using namespace std;
/*
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
*/
#define TRACE
#ifdef TRACE
#define trace(...) { cerr<<"[ "; __trace__(#__VA_ARGS__, __VA_ARGS__);}
#undef endl
template <typename Arg1,typename Arg2>
ostream& operator << (ostream& out, const pair<Arg1,Arg2> &x) {
    return out<<"("<<x.first<<","<<x.second<<")";
}
 
template <typename Arg1>
ostream& operator << (ostream& out, const vector<Arg1> &a) {
    out<<"[";for(const auto &x:a)out<<x<<",";return out<<"]";
}
 
template <typename Arg1>
ostream& operator << (ostream& out, const set<Arg1> &a) {
    out<<"[";for(const auto &x:a)out<<x<<",";return out<<"]";
}
 
template <typename Arg1,typename Arg2>
ostream& operator << (ostream& out, const map<Arg1,Arg2> &a) {
    out<<"[";for(const auto &x:a)out<<x<<",";return out<<"]";
}
 
template <typename Arg1>
void __trace__(const string name, Arg1&& arg1){
    cerr << name << " : " << arg1 << " ] " << endl;
}
 
template <typename Arg1, typename... Args>
void __trace__(const string names, Arg1&& arg1, Args&&... args){
    const string name = names.substr(0,names.find(','));
    cerr<<name<<" : "<<arg1<<" | ";
    __trace__(names.substr(1+(int)name.size()), args...);
}
#else
    #define trace(args...)
#endif
 
typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<pair<int,int>> vii;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<pair<ll,ll>> vll;
typedef vector<vl> vvl;
//typedef tree<pii, null_type, less<pii>, rb_tree_tag, tree_order_statistics_node_update> oset;
#define fori(i,n) for(int i=0;i<n;i++)
#define rep(i,a,b) for(int i=a;i<=b;i++)
#define repd(i,a,b) for(int i=a;i>=b;i--)
#define ford(i,n)  for(int i=n-1;i>=0;i--)
#define all(x) x.begin(),x.end()
#define pb push_back
#define endl '\n'
#define sz(a) (int)a.size()
#define fi first
#define se second
clock_t time_p=clock();
void time_taken()
{
    time_p=clock()-time_p;
    cerr<<"Time Taken : "<<(float)(time_p)/CLOCKS_PER_SEC<<"\n";
}
//const ll mod=998244353;
const ll mod=1e9+7;
const ll INF=1e18;
ll get(vi a,vi b,int ind,vector<bool> used){
    if(sz(b) == 0) return 1;
    if(ind == sz(a)) {
        bool ok=1;
        fori(i,sz(b)){
            ok &= (used[i]==1);
        }
        return ok;
    }
    ll ans = 0;
    ans+=get(a,b,ind+1,used);
    ans%=mod;
    fori(i,sz(b)){
        if(used[i] == 0 and a[ind] < b[i]){
            used[i] = 1;
            ans += get(a,b,ind+1,used);
            ans %= mod;
            used[i] = 0;
        }
    } 
    return ans;
}
int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);cout.tie(NULL);
    cout.precision(12);cout << fixed;
    int n;
    cin >> n;
    vi a(n);
    fori(i,n) cin >> a[i];
    vi adj[n];
    fori(i,n){
        adj[a[i]].pb(i);
    }
    int cnt = sz(adj[0]);
    rep(i,1,n-1){
        if(cnt > 3 or sz(adj[i]) > 3 or (sz(adj[i-1]) < sz(adj[i]))){
            cout << 0 << endl;
            return 0;
        }
    }
    if(a[0]!=0){
        cout << 0 << endl;
        return 0;
    }
    ll ans;
    if(cnt==1) ans=3;
    else ans=6;
    for(int i=1;i < n and sz(adj[i]);i++){
        vector<bool> used(sz(adj[i]),0);
        ans = ans * get(adj[i-1],adj[i],0,used)%mod;
    }
    cout << ans << endl;
    time_taken();
    return 0;
}

Measured memory_bytes: 6144