P1403 [AHOI2005]约数研究

很简单的数学题啦推一下就知道f(i) = n / i啦
AC代码：
 1 #include<bits/stdc++.h>
 2 #define pi acos(-1)
 3 typedef long long ll;
 4 typedef unsigned long long ull;
 5 using namespace std;
 6 
 7 namespace io {
 8     const int SIZE = 1e7 + 10;
 9     char inbuff[SIZE];
10     char *l, *r;
11     inline void init() {
12         l = inbuff;
13         r = inbuff + fread(inbuff, 1, SIZE, stdin);
14     }
15     inline char gc() {
16         if (l == r) init();
17         return (l != r) ? *(l++) : EOF;
18     }
19     void read(int &x) {
20         x = 0; char ch = gc();
21         while(!isdigit(ch)) ch = gc();
22         while(isdigit(ch)) x = x * 10 + ch - '0', ch = gc();
23     }
24 } using io::read;
25 
26 bool cmp(const int &a, const int &b){
27     return a > b;
28 }
29 
30 int main(){
31     ios::sync_with_stdio(false);
32     int n;
33     cin>>n;
34     int ans = 0;
35     for (int i = 1; i <= n; i++) ans += n / i;
36     cout<<ans<<endl;
37     return 0;
38 }