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。
1 #include <iostream> 2 #include <queue> 3 #include <stack> 4 #include <cstdio> 5 #include <vector> 6 #include <map> 7 #include <set> 8 #include <bitset> 9 #include <algorithm> 10 #include <cmath> 11 #include <cstring> 12 #include <cstdlib> 13 #include <string> 14 #include <sstream> 15 #define x first 16 #define y second 17 #define pb push_back 18 #define mp make_pair 19 #define lson l,m,rt*2 20 #define rson m+1,r,rt*2+1 21 #define mt(A,B) memset(A,B,sizeof(A)) 22 #define mod 1000000007 23 using namespace std; 24 typedef long long LL; 25 const int N=200000+10; 26 const LL INF=0x3f3f3f3f3f3f3f3fLL; 27 int ans=0; 28 int f(int n) 29 { 30 if(n<1)return 0; 31 else if(n>=1&&n<=9)return 1; 32 else 33 { 34 int bit=0,highbit=n; 35 while(highbit>=10) 36 { 37 highbit/=10; 38 bit++; 39 } 40 int p=pow(10,bit); 41 if(highbit==1) 42 { 43 return f(p-1)+f(n-p)+n-p+1; 44 } 45 else 46 { 47 return highbit*f(p-1)+f(n-highbit*p)+p; 48 } 49 } 50 } 51 int main() 52 { 53 #ifdef Local 54 freopen("data.txt","r",stdin); 55 #endif 56 int n; 57 cin>>n; 58 cout<<f(n)<<endl; 59 }