面试宝典——字符串字数统计问题

#include"iostream"
#include"stdio.h"
using namespace std;
typedef long long ll;

ll f(ll n)
{
    ll fn=0,ntemp=n;
    ll step;
    for(step=1;ntemp>0;step*=10,ntemp/=10)
    {
        fn+=(((ntemp-1)/10)+1)*step;
        if((ntemp%10)==1)
        {
            fn-=step-(n%step+1);
        }
    }
    return fn;
}

ll get_max_fn_equal_n(ll upper_bound)

{
    ll n=1,fn=0;
    ll max=1;
    while(n<=upper_bound)
    {
        fn=f(n);
        if(fn==n)
        {
            max=n;
            cout<<n++<<endl;
        }
        else if(fn<n)
            n+=(n-fn)/10+1;
        else
            n=fn;
    }
    return max;
}

int main()
{
    ll upper_bound=4000000000;
    cout<<f(13)<<endl;
    cout<<get_max_fn_equal_n(upper_bound);
    return 0;
}