Submission #1835542

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Source Code Expand

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define fbo find_by_order
#define ook order_of_key

typedef long long ll;
typedef pair<ll,ll> ii;
typedef vector<int> vi;
typedef long double ld; 
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> pbds;
typedef set<int>::iterator sit;
typedef map<int,int>::iterator mit;
typedef vector<int>::iterator vit;
const int MOD = 1e9 + 7;
int dp[3011][3011][2];

void add(int &a, int b)
{
	a+=b;
	while(a>=MOD) a-=MOD;
}

int main()
{
	ios_base::sync_with_stdio(0); cin.tie(0);
	ll n, m; cin>>n>>m;
	for(int i=0;i<=n;i++) 
	{
		dp[0][i][i==0] = 1;
	}
	for(int i=0;i<m;i++)
	{
		for(int j=0;j<=n;j++)
		{
			for(int k=0;k<2;k++)
			{
				int v=dp[i][j][k];
				if(v==0) continue;
				//RR
				if(j>=1)
				{
					add(dp[i+1][j-1][k|(j-1==0)],v);
				}
				//BB
				if(j+1<=n)
				{
					add(dp[i+1][j+1][k|(j==0)],v);
				}
				//RB
				if(j-1>=0)
				{
					add(dp[i+1][j][k|(j-1==0)],v);
				}
				//BR
				if(j+1<=n)
				{
					add(dp[i+1][j][k|(j==0)],v);
				}
			}
		}
	}
	int ans=0;
	for(int i=0;i<=n;i++)
	{
		add(ans,dp[m][i][1]);
	}
	cout<<ans<<'\n';
}

Submission Info

Judge Result