【数位dp】P4999 烦人的数学作业

数位dp的基础题，复习模板用！！

代码

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll mod=1e9+7;
ll f[20][200],a[20];
ll dfs(int x,int sum,int top)
{
	if(!x) return sum;
	if(!top && f[x][sum]>=0) return f[x][sum];
	int up=9; if(top) up=a[x];
	ll res=0;
	for(int i=0;i<=up;i++)
		res=(res+dfs(x-1,i+sum,top&&(i==up)))%mod;
	if(!top) f[x][sum]=res;
	return res;
}
ll calc(ll x)
{
	int sum=0;
	while(x)
	{
		a[++sum]=x%10;
		x/=10;
	}
	return dfs(sum,0,1)%mod;
}
int main()
{
	freopen("a.in","r",stdin);
	freopen("a.out","w",stdout);
	int t;ll x,y;
	scanf("%d",&t);
	memset(f,-1,sizeof(f));
	while(t--)
	{
		scanf("%lld%lld",&x,&y);
		printf("%lld\n",(calc(y)-calc(x-1)+mod)%mod);
	}
	return 0;
}