Codeforces 466 D. Increase Sequence

一道不错的想法题。。。。。

D. Increase Sequence

time limit per test

1 second

memory limit per test

256 megabytes

input

standard input

output

standard output

Peter has a sequence of integers a1, a2, ..., an. Peter wants all numbers in the sequence to equal h. He can perform the operation of "adding one on the segment [l, r]": add one to all elements of the sequence with indices from l to r (inclusive). At that, Peter never chooses any element as the beginning of the segment twice. Similarly, Peter never chooses any element as the end of the segment twice. In other words, for any two segments [l1, r1] and [l2, r2], where Peter added one, the following inequalities hold: l1 ≠ l2 and r1 ≠ r2.

How many distinct ways are there to make all numbers in the sequence equal h? Print this number of ways modulo 1000000007 (109 + 7). Two ways are considered distinct if one of them has a segment that isn't in the other way.

Input

The first line contains two integers n, h (1 ≤ n, h ≤ 2000). The next line contains n integers a1, a2, ..., an (0 ≤ ai ≤ 2000).

Output

Print a single integer — the answer to the problem modulo 1000000007 (109 + 7).

Sample test(s)

input

3 2
1 1 1

output

4

input

5 1
1 1 1 1 1

output

1

input

4 3
3 2 1 1

output

0

#include 
#include 
#include 
#include 

using namespace std;

typedef long long int LL;

const LL maxn=2200,MOD=1e9+7;

LL a[maxn],b[maxn];

int main()
{
	int n,h;
	cin>>n>>h;
	for(int i=1;i<=n;i++) 
	{
		cin>>a[i];
		a[i]=h-a[i];
	}
	for(int i=1;i<=n+1;i++)
	{
		b[i]=a[i]-a[i-1];
	}
	LL ans=1,cnt=0;
	for(int i=1;i<=n+1;i++)
	{
		if(b[i]==1)
		{
			cnt++;
		}
		else if(b[i]==0)
		{
			if(cnt) ans=(ans*(cnt+1))%MOD;
		}
		else if(b[i]==-1)
		{
			ans=(ans*cnt)%MOD;
			cnt--;
		}
		else ans=0;
	}
	cout<