Problem:
Score : 900 points

Problem Statement

You are developing frog-shaped robots, and decided to race them against each other.

First, you placed N robots onto a number line.
These robots are numbered 1 through N.
The current coordinate of robot i is x_i.
Here, all x_i are integers, and 0 < x_1 < x_2 < ... < x_N.

You will repeatedly perform the following operation:

Select a robot on the number line. Let the coordinate of the robot be x. Select the destination coordinate, either x-1 or x-2, that is not occupied by another robot. The robot now jumps to the selected coordinate.

When the coordinate of a robot becomes 0 or less, the robot is considered finished and will be removed from the number line immediately.
You will repeat the operation until all the robots finish the race.

Depending on your choice in the operation, the N robots can finish the race in different orders.
In how many different orders can the N robots finish the race?
Find the answer modulo 10^9+7.

Constraints

2 ≤ N ≤ 10^5

x_i is an integer.

0 < x_1 < x_2 < ... < x_N ≤ 10^9

Partial Score

In a test set worth 500 points, N ≤ 8.

Input

The input is given from Standard Input in the following format:

N
x_1 x_2 ... x_N

Output

Print the number of the different orders in which the N robots can finish the race, modulo 10^9+7.

Sample Input 1

3
1 2 3

Sample Output 1

4

There are four different orders in which the three robots can finish the race:

(Robot 1 → Robot 2 → Robot 3)

(Robot 1 → Robot 3 → Robot 2)

(Robot 2 → Robot 1 → Robot 3)

(Robot 2 → Robot 3 → Robot 1)

Sample Input 2

3
2 3 4

Sample Output 2

6

There are six different orders in which the three robots can finish the race:

(Robot 1 → Robot 2 → Robot 3)

(Robot 1 → Robot 3 → Robot 2)

(Robot 2 → Robot 1 → Robot 3)

(Robot 2 → Robot 3 → Robot 1)

(Robot 3 → Robot 1 → Robot 2)

(Robot 3 → Robot 2 → Robot 1)

For example, the order (Robot 3 → Robot 2 → Robot 1) can be achieved as shown in the figure below:

Sample Input 3

8
1 2 3 5 7 11 13 17

Sample Output 3

10080

Sample Input 4

13
4 6 8 9 10 12 14 15 16 18 20 21 22

Sample Output 4

311014372

Remember to print the answer modulo 10^9+7.
This case is not included in the test set for the partial score.

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Runtime.CompilerServices;
using System.Text;
using static Template;
using static System.Console;
using static System.Convert;
using static System.Math;
using Pi = Pair<int, int>;
using Number = System.Int64;

class Solver
{
    Scanner sc = new Scanner();
    //
    public void Solve()
    {
        var N = sc.Int;
        var ar = sc.ArrInt;
        ModInt res = 1;
        var ct = 0;
        for(var i = 0; i < N; i++)
        {
            ct++;
            if(ar[i]<2*ct-1)
            {
                res *= ct;
                ct--;
            }
        }
        for (var i = 1; i <= ct; i++)
            res *= i;
        WriteLine(res);
    }
}

public struct ModInt
{
    public const long MOD = (int)1e9 + 7;
    //public const long MOD = 998244353;
    public long num { get; set; }
    public ModInt(long n = 0) { num = n; }
    private static ModInt[] _fac;//階乗
    private static ModInt[] _inv;//逆数
    private static ModInt[] _facrev;//1/(i!)
    public override string ToString()
        => num.ToString();
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt operator +(ModInt l, ModInt r)
    {
        l.num += r.num;
        if (l.num >= MOD) l.num -= MOD;
        return l;
    }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt operator -(ModInt l, ModInt r)
    {
        l.num -= r.num;
        if (l.num < 0) l.num += MOD;
        return l;
    }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt operator *(ModInt l, ModInt r)
        => new ModInt(l.num * r.num % MOD);
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt operator /(ModInt l, ModInt r)
        => l * Pow(r, MOD - 2);
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static implicit operator long(ModInt l)
        => l.num;
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static implicit operator ModInt(long n)
    {
        n %= MOD; if (n < 0) n += MOD;
        return new ModInt(n);
    }

    public static ModInt Pow(ModInt m, long n)
    {
        if (n == 0) return 1;
        if (n % 2 == 0) return Pow(m * m, n >> 1);
        else return Pow(m * m, n >> 1) * m;
    }

    public static void CombBuild(int n)
    {
        _fac = new ModInt[n + 1];
        _facrev = new ModInt[n + 1];
        _inv = new ModInt[n + 1];
        _inv[1] = 1;
        _fac[0] = _fac[1] = 1;
        _facrev[0] = _facrev[1] = 1;
        for (var i = 2; i <= n; i++)
        {
            _fac[i] = _fac[i - 1] * i;
            _inv[i] = MOD - _inv[MOD % i] * (MOD / i);
            _facrev[i] = _facrev[i - 1] * _inv[i];
        }
    }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt Fac(ModInt n)
        => _fac[n];
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt Div(ModInt n)
        => _inv[n];
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt FacRev(ModInt n)
        => _facrev[n];
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static ModInt Comb(ModInt n, ModInt r)
    {
        if (n < r) return 0;
        if (n == r) return 1;
        var calc = _fac[n];
        calc = calc * _facrev[r];
        calc = calc * _facrev[n - r];
        return calc;
    }
}
#region Template
public class Template
{
    static void Main(string[] args)
    {
        Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
        new Solver().Solve();
        Console.Out.Flush();
    }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static bool chmin<T>(ref T num, T val) where T : IComparable<T>
    { if (num.CompareTo(val) == 1) { num = val; return true; } return false; }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static bool chmax<T>(ref T num, T val) where T : IComparable<T>
    { if (num.CompareTo(val) == -1) { num = val; return true; } return false; }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static void swap<T>(ref T v1, ref T v2)
    { var t = v2; v2 = v1; v1 = t; }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static T[] Create<T>(int n, Func<T> f)
    {
        var rt = new T[n];
        for (var i = 0; i < rt.Length; ++i)
            rt[i] = f();
        return rt;
    }
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public static T[] Create<T>(int n, Func<int, T> f)
    {
        var rt = new T[n];
        for (var i = 0; i < rt.Length; ++i)
            rt[i] = f(i);
        return rt;
    }
    public static void Fail<T>(T s) { Console.WriteLine(s); Console.Out.Close(); Environment.Exit(0); }
}

public class Scanner
{
    public string Str => ReadLine().Trim();
    public int Int => int.Parse(Str);
    public long Long => long.Parse(Str);
    public double Double => double.Parse(Str);
    public int[] ArrInt => Str.Split(' ').Select(int.Parse).ToArray();
    public long[] ArrLong => Str.Split(' ').Select(long.Parse).ToArray();
    public char[][] Grid(int n) => Create(n, () => Str.ToCharArray());
    public int[] ArrInt1D(int n) => Create(n, () => Int);
    public long[] ArrLong1D(int n) => Create(n, () => Long);
    public int[][] ArrInt2D(int n) => Create(n, () => ArrInt);
    public long[][] ArrLong2D(int n) => Create(n, () => ArrLong);
    public Pair<T1, T2> PairMake<T1, T2>() => new Pair<T1, T2>(Next<T1>(), Next<T2>());
    public Pair<T1, T2, T3> PairMake<T1, T2, T3>() => new Pair<T1, T2, T3>(Next<T1>(), Next<T2>(), Next<T3>());
    private Queue<string> q = new Queue<string>();
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public T Next<T>() { if (q.Count == 0) foreach (var item in Str.Split(' ')) q.Enqueue(item); return (T)Convert.ChangeType(q.Dequeue(), typeof(T)); }
    public void Make<T1>(out T1 v1) => v1 = Next<T1>();
    public void Make<T1, T2>(out T1 v1, out T2 v2) { v1 = Next<T1>(); v2 = Next<T2>(); }
    public void Make<T1, T2, T3>(out T1 v1, out T2 v2, out T3 v3) { Make(out v1, out v2); v3 = Next<T3>(); }
    public void Make<T1, T2, T3, T4>(out T1 v1, out T2 v2, out T3 v3, out T4 v4) { Make(out v1, out v2, out v3); v4 = Next<T4>(); }
    public void Make<T1, T2, T3, T4, T5>(out T1 v1, out T2 v2, out T3 v3, out T4 v4, out T5 v5) { Make(out v1, out v2, out v3, out v4); v5 = Next<T5>(); }
    public void Make<T1, T2, T3, T4, T5, T6>(out T1 v1, out T2 v2, out T3 v3, out T4 v4, out T5 v5, out T6 v6) { Make(out v1, out v2, out v3, out v4, out v5); v6 = Next<T6>(); }
}
public class Pair<T1, T2> : IComparable<Pair<T1, T2>>
{
    public T1 v1;
    public T2 v2;
    public Pair() { }
    public Pair(T1 v1, T2 v2)
    { this.v1 = v1; this.v2 = v2; }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public int CompareTo(Pair<T1, T2> p)
    {
        var c = Comparer<T1>.Default.Compare(v1, p.v1);
        if (c == 0)
            c = Comparer<T2>.Default.Compare(v2, p.v2);
        return c;
    }
    public override string ToString()
        => $"{v1.ToString()} {v2.ToString()}";
    public void Deconstruct(out T1 a, out T2 b) { a = v1; b = v2; }
}

public class Pair<T1, T2, T3> : Pair<T1, T2>, IComparable<Pair<T1, T2, T3>>
{
    public T3 v3;
    public Pair() : base() { }
    public Pair(T1 v1, T2 v2, T3 v3) : base(v1, v2)
    { this.v3 = v3; }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public int CompareTo(Pair<T1, T2, T3> p)
    {
        var c = base.CompareTo(p);
        if (c == 0)
            c = Comparer<T3>.Default.Compare(v3, p.v3);
        return c;
    }
    public override string ToString()
        => $"{base.ToString()} {v3.ToString()}";
    public void Deconstruct(out T1 a, out T2 b, out T3 c) { Deconstruct(out a, out b); c = v3; }
}
#endregion