P1384 幸运数与排列

#include<iostream>
#include<cstdio>
#include<cstring>
#include<vector>
#include<algorithm>
using namespace std;
long long fac[14]={1,1,2,6,24,120,720,5040,40320,362880,3628800,39916800,479001600,6227020800};
int n,k,x,e[12],a[12];
vector <int> p1,p2;
int dfs(int d,int w,int z){//普通的数位dp
    if(!d) return !z;
    if(!w&&!z&&e[d]>-1) return e[d];
    int lim=w?a[d]:9,tot=0;
    for(int i=0;i<=lim;++i)
        if(i==4||i==7||(z&&!i))
            tot+=dfs(d-1,w&&(i==lim),z&&!i);
    if(!w&&!z) e[d]=tot;
    return tot;
}
int solve1(int A){
    int t=0;
    while(A) a[++t]=A%10,A/=10;
    return dfs(t,1,1);
}
bool is(int x){
    for(;x;x/=10) if(x%10!=4&&x%10!=7) return 0;
    return 1;
}
int solve2(){
    --k;
    for(int i=n-x+1;i<=n;++i) p1.push_back(i);
    for(int i=x,v;i>=1;--i){//逆康托展开求原串
        v=k/fac[i-1]; k%=fac[i-1];
        sort(p1.begin(),p1.end());
        p2.push_back(p1[v]);
        p1.erase(p1.begin()+v);
    }int tot=0;
    for(int i=0;i<x;++i)
        if(is(n-x+i+1)&&is(p2[i])) ++tot;
    return tot;
}
int main(){
    scanf("%d%d",&n,&k);
    if(n<13&&k>fac[n]){printf("-1");return 0;}
    while(k>fac[x])++x;
    memset(e,-1,sizeof(e));
    printf("%d",solve1(n-x)+solve2());
    return 0;
}