题解 | #明七暗七#

写一个灵神板子

#include<bits/stdc++.h>
using namespace std;
using ll=long long;
#define int long long
int dp[17][23][3];

int solve(int a){
	string s=to_string(a);
	int n=s.size();
	memset(dp,-1,sizeof(dp));
	function<int(int,int,int,bool)>dfs=[&](int idx,int val,int f,bool lim)->int{
		if(idx==n)return (f||!val)?1:0;
		if(!lim&&dp[idx][val][f]!=-1)return dp[idx][val][f];
		int res=0;
		for(int d=0,up=lim?s[idx]-'0':9;d<=up;++d){
			res+=dfs(idx+1,(val*10+d)%7,f||(d==7),lim&&d==up);
		}
		if(!lim)dp[idx][val][f]=res;
		return res;
	};
	return dfs(0,0,0,true);
}

signed main(void){
	ios::sync_with_stdio(0),cin.tie(0),cout.tie(0);
	int a;cin>>a;
	int k;cin>>k;
	int low=solve(a);
	int l=a+1,r=4e12;
	int ans=-1;
	while(l<=r){
		int mid=l+r>>1;
		int t=solve(mid);
		if(t-low>=k){
			ans=mid;
			r=mid-1;
		}else l=mid+1;
	}
	cout<<ans<<"\n";
	return 0;
}