题解

#include<iostream>
using namespace std;
bool valid(long long n){
    int a[10]={0,0,0,0,0,0,0,0,0,0};
    if(n<10000000){
        a[0]++;
    }
    for(;n>0;n/=10){
        a[n%10]++;
        if(a[n%10]>1){
            return false;
        }
    }
    return true;
}
int main(){
    long long n;
    int c=0;
    cin>>n;
    long long p = 9*8*7*6*5*4*3;
    long long left =  n/10000000;
    c += left * p;
    for(long long i=left * 10000000;i<=n;i++){
        long long first, second, third;
        first = i/10000000;
        second = (i - first * 10000000)/1000000;
        if(first == second) {
            i += 999999;
            continue;
        }
        if(!valid(i)){
            continue;
        }
        c++;
    }
    cout<<c;
}