[Project Euler] Problem 21

Let d(n) be defined as the sum of proper divisors of n (numbers less than n which divide evenly into n).
If d(a) = b and d(b) = a, where a b, then a and b are an amicable pair and each of a and b are called amicable numbers.

For example, the proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110; therefore d(220) = 284. The proper divisors of 284 are 1, 2, 4, 71 and 142; so d(284) = 220.

Evaluate the sum of all the amicable numbers under 10000.

我们直接编写函数d(n)，如果d(d(n))=n,并且n!=d(n)那么n就是符合要求的数

我们直接暴力搜索

#include < iostream >
#include < cmath >
using namespace std;

int d( int tmp){
int sum = 0 ;
int i;
for (i = 1 ; i < sqrt(tmp); i ++ ){
if (tmp % i == 0 ){
sum = sum + i + tmp / i;
}
}
if (i * i == tmp){
sum += i;
}
return sum - tmp;
}

int main(){
int sum = 0 ;
for ( int i = 1 ; i < 10000 ; i ++ ){
if (i == d(d(i)) && i != d(i)){
sum += i;
}
}
cout << sum << endl;
return 0 ;
}