Luogu P4124 [CQOI2016]手机号码 题解

#include<bits/stdc++.h>
#define int long long 
using namespace std;
int a[15],m,f[15][10][10][2][2][2][2][2];
inline int DFS(int x,int las,int lasqr,int has,int is4,int is8,int lim,int st){
    if(is4&&is8) return 0;
    if(x==0) return has;
    if(~f[x][las][lasqr][has][is4][is8][lim][st]) return f[x][las][lasqr][has][is4][is8][lim][st];
    int Max=lim?a[x]:9,res=0;
    for(int i=0;i<=Max;i++)
        if(st==1) res+=DFS(x-1,i,las,has,is4(i==4),is8(i==8),lim&&(i==Max),st&&(i==0));
        else res+=DFS(x-1,i,las,has(las==lasqr&&las==i),is4(i==4),is8(i==8),lim&&(i==Max),st&&(i==0));
    return f[x][las][lasqr][has][is4][is8][lim][st]=res;
}
inline int solve(int n){
    m=0;memset(f,-1,sizeof(f));
    while(n){
        a[++m]=n%10;
        n/=10;
    }
    return DFS(m,-1,-1,0,0,0,1,1);
}
int L,R;
signed main(){
    scanf("%lld%lld",&L,&R);
    printf("%lld\n",solve(R)-solve(L-1));
}