2016-2017 ACM-ICPC, NEERC, Central Subregional Contest H

Non-random numbers

题意：求有多少个满足第i为数不为i且不含前导0的n位数

思路：第1位有8种情况(除了0，1)，2-9位有9种情况，10位以上每位有10种情况，大于9以后只要往后面加0就是了，或者用大数乘的模板也可以

AC代码：

#include "iostream"
#include "string.h"
#include "stack"
#include "queue"
#include "string"
#include "vector"
#include "set"
#include "map"
#include "algorithm"
#include "stdio.h"
#include "math.h"
#define ll long long
#define bug(x) cout<<x<<" "<<"UUUUU"<<endl;
#define mem(a) memset(a,0,sizeof(a))
using namespace std;
const int N=1e5+100;
/////2016-2017 ACM-ICPC, NEERC, Central Subregional Contest
string Multiply(string s,int x)
{
    reverse(s.begin(),s.end());
    int cmp=0;
    for(int i=0;i<s.size();i++)
    {
        cmp=(s[i]-'0')*x+cmp;
        s[i]=(cmp%10+'0');
        cmp/=10;
    }
    while(cmp)
    {
        s+=(cmp%10+'0');
        cmp/=10;
    }
    reverse(s.begin(),s.end());
    return s;
}

string ans="8";
int main(){
    freopen("input.txt","r",stdin);
    freopen("output.txt","w",stdout);
    cin>>n;
    for(int i=2; i<=n; ++i){
        if(i<10) ans=Multiply(ans,9);
        else ans=Multiply(ans,10);
    }
    cout<<ans;
    return 0;
}