链接

如果数位dp一直是这个难度，我将永远和数位dp告别

我记得有篇博客叫做“刘聪浅谈数位dp”

里面有幅图，挺好看的

很久之前写的了代码

忘了为什么要这么做

QwQ

#include<bits/stdc++.h>
#define re return 
#define reg register 
#define ll long long 
#define inc(i,l,r) for(int i=l;i<=r;++i)
#define dec(i,l,r) for(int i=l;i>=r;--i)
using namespace std;
ll l,r,k,b,f[44][44],num[44];

void vivi()
{
    f[0][0]=1;
    inc(i,1,31){
        f[i][0]=f[i-1][0];
        inc(j,1,i)
            f[i][j]=f[i-1][j]+f[i-1][j-1];
    }
} 

ll solve(ll x)
{
    int len=0;
    while(x){
        num[++len]=x%b;
        x/=b;
    }
    
    ll ans=0,cnt=0;
    dec(i,len,1)
    if(num[i]==1)
    {
        ans+=f[i-1][k-cnt];
        ++cnt;
        if(cnt==k)re ans+1;
    }
    else if(num[i]>1)re ans+f[i][k-cnt];
    
    re ans;
}

int main()
{
//    freopen("in.txt","r",stdin);
    scanf("%lld%lld%lld%lld",&l,&r,&k,&b);
    vivi();
    printf("%lld ",solve(r)-solve(l-1));
    
    re 0;
}