Submission #1695452

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Source Code Expand

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
const int maxn=3e3+10;
const int mod=1e9+7;
int dp[maxn<<1][maxn][2];
inline int read(){
	int x=0,flag=1;
	char ch=getchar();
	while(!isdigit(ch) && ch!='-')ch=getchar();
	if(ch=='-')flag=-1,ch=getchar();
	while(isdigit(ch))x=(x<<3)+(x<<1)+ch-'0',ch=getchar();
	return x*flag;
}
int n,sum=0;
int add(int &x,int y){
	while(y>=mod)y-=mod;
	x+=y;
	while(x>=mod)x-=mod;
}
int main(){
	int i,j,k,m,p;
	n=read();m=read();
	int pos=1;
	for(i=1;i<=n;i++)dp[0][i][0]=1;
	dp[0][0][1]=1;
	for(i=0;i<m;i++)
		for(j=0;j<=n;j++)
			for(k=0;k<=1;k++){
				if(j>0){
					add(dp[i+1][j-1][k|(j==1)],dp[i][j][k]);
					add(dp[i+1][j][k|(j==1)],dp[i][j][k]);
				}
				if(n-j>0){
					add(dp[i+1][j][k],dp[i][j][k]);
					add(dp[i+1][j+1][k],dp[i][j][k]);
				}
			}
	int ans=0;
	for(i=0;i<=n;i++)ans=(ans+dp[m][i][1])%mod;
	printf("%d\n",ans);
	return 0;
}

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