1009 数字1的数量

思路：当初做的时候YY了好久一直YY不出来，查了题解发现好多种解法，学了一种最简单的，确实是一道不错的题，以下截图自其它大牛博客。

#include <iostream>
#include <queue>
#include <stack>
#include <cstdio>
#include <vector>
#include <map>
#include <set>
#include <bitset>
#include <algorithm>
#include <cmath>
#include <cstring>
#include <cstdlib>
#include <string>
#include <sstream>
#define x first
#define y second
#define pb push_back
#define mp make_pair
#define lson l,m,rt*2
#define rson m+1,r,rt*2+1
#define mt(A,B) memset(A,B,sizeof(A))
#define mod 1000000007
using namespace std;
typedef long long LL;
const int N=200000+10;
const LL INF=0x3f3f3f3f3f3f3f3fLL;
int ans=0;
int f(int n)
{
    if(n<1)return 0;
    else if(n>=1&&n<=9)return 1;
    else
    {
        int bit=0,highbit=n;
        while(highbit>=10)
        {
            highbit/=10;
            bit++;
        }
        int p=pow(10,bit);
        if(highbit==1)
        {
            return f(p-1)+f(n-p)+n-p+1;
        }
        else
        {
            return highbit*f(p-1)+f(n-highbit*p)+p;
        }
    }
}
int main()
{
#ifdef Local
    freopen("data.txt","r",stdin);
#endif
    int n;
    cin>>n;
    cout<<f(n)<<endl;
}