[CQOI2016] 手机号码 - 数位dp

Description

求 \([l,r]\) 中至少有 \(3\) 个连续相同数字，不同时有 \(4,8\) 的数字个数。

Solution

正常数位 dp 即可，注意前导零特判

#include <bits/stdc++.h>
using namespace std;

#define int long long 
const int N = 1000005;

vector <int> a;

int f[20][5][10][4];

int solve(int pos,int con,int last,int flag,int full)
{
    if(pos<0) return con==3;
    if(!full && ~f[pos][con][last][flag]) return f[pos][con][last][flag];

    int lim=full?a[pos]:9;
    int ans=0;
    
    for(int now=(con==0?1:0);now<=lim;now++)
    {
        int newpos=pos-1;
        int newcon=(now==last?con:0)+1;
        if(con>=3) newcon=3;
        int newlast=now;
        int newflag=flag|(2*(now==8))|(now==4);
        if(newflag==3) continue;
        int newfull=full&&now==lim;
        ans+=solve(newpos,newcon,newlast,newflag,newfull);
    }

    if(!full) f[pos][con][last][flag]=ans;
    return ans;
}

int solve(int x)
{
    a.clear();
    memset(f,-1,sizeof f);
    while(x) 
    {
        a.push_back(x%10);
        x/=10;
    }
    return solve(a.size()-1,0,0,0,1);
}

signed main()
{
    ios::sync_with_stdio(false);

    int l,r;
    cin>>l>>r;
    --l;
    if(l>=(int)(1e10+0.5)) cout<<solve(r)-solve(l)<<endl;
    else cout<<solve(r)<<endl;

    return 0;
}