C. Shaass and Lights

http://codeforces.com/contest/294/problem/C

代码：

#include<iostream>
#include<stdio.h>
#include<string.h>
#include<math.h>
#include<algorithm>
#include<vector>
#include<set>
#include<queue>
#include<map>
#include<string>
#include <iomanip>

using namespace std;

const int INF=0x3f3f3f3f;
const long long MOD=1000000007;
const int N=2005;
long long a[N];
vector<int>vt;
long long c[N][N];
int main()
{
    //freopen("data.in","r",stdin);
    for(int i=0;i<N;++i)
    for(int j=0;j<=i;++j)
    {
        if(j==0) {c[i][j]=1;continue;}
        if(i==j) {c[i][j]=1;continue;}
        c[i][j]=(c[i-1][j-1]+c[i-1][j])%MOD;
    }
    a[0]=1;
    for(int i=1;i<N;++i)
    a[i]=(a[i-1]*2)%MOD;
    int n,m;
    while(cin>>n>>m)
    {
        vt.clear();
        for(int i=1;i<=m;++i)
        {
            int tmp;
            cin>>tmp;
            vt.push_back(tmp);
        }
        sort(vt.begin(),vt.end());
        long long ans=1;
        for(unsigned int i=1;i<m;++i)
        {
            int k=vt[i]-vt[i-1]-1;
            if(k>1)
            ans=(ans*a[k-1])%MOD;
        }
        vt.insert(vt.begin(),0);
        vt.push_back(n+1);
        int sum=0;
        for(unsigned int i=0;i<vt.size();++i)
        {
            int k=0;
            if(i>0)
            k=vt[i]-vt[i-1]-1;
            if(sum>0&&k>0)
            ans=(ans*c[sum+k-1+1][k])%MOD;
            sum+=k;
        }
        cout<<ans<<endl;
    }
    return 0;

}