具體題解發一下大佬的分析http://blog.csdn.net/wyg1997/article/details/52169036

 1 #include <iostream>
 2 #include <algorithm>
 3 using namespace std;
 4 typedef long long LL;
 5 
 6 int main(){
 7     std::ios::sync_with_stdio(false);
 8     int n;
 9     cin >> n;
10     LL ans = 0;
11     int
 
 tn = n;
12     int mu = 1;
13     while (tn)
14     {
15         int t = tn % 10;
16         if (t == 0)
17             ans += (n / (mu * 10))*mu;
18         else if (t == 1)
19         {
20             ans += (n / (mu * 10))*mu;
21             ans += (n%mu) + 1;
22         }
23         else
24
 
             ans += (n / (mu * 10) + 1)*mu;
25         tn /= 10;
26         mu *= 10;
27     }
28     cout << ans << endl;
29     return 0;
30 }

51Nod 1009 數字1的數量