2020牛客多校六 H Harmony Pairs

https://ac.nowcoder.com/acm/contest/5671/H
题意：
思路：

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn=350;
const ll mod=1e9+7;

char s[105];
int d[105],n;
ll dp[105][2005][2][2];

ll dfs(int x,int dif,bool limb,bool p)
{	
	if(x>n)return dif>1000;
	if(dp[x][dif][limb][p]!=-1)return dp[x][dif][limb][p];
	ll tmp=0;
	for(int b=0;b<=(limb?d[x]:9);b++)
		for(int a=0;a<=(p?b:9);a++)
			tmp=(tmp+dfs(x+1,dif+a-b,limb&&(b==d[x]),p&&(a==b)))%mod;
	return dp[x][dif][limb][p]=tmp;
}

int main()
{
	//freopen("input.in","r",stdin);
	cin>>s+1;
	n=strlen(s+1);
	for(int i=1;i<=n;i++)d[i]=s[i]-'0';
	memset(dp,-1,sizeof(dp));
	cout<<dfs(1,1000,1,1)<<endl;
	return 0;
}