001~003

 1 #include <iostream>
 2 
 3 using namespace std;
 4 
 5 int main()
 6 {
 7     int sum = 0;
 8     for (int i = 3; i < 1000; ++ i) {
 9         if (i % 3 == 0 || i % 5 == 0) {
10             sum += i;
11         }
12     }
13     cout << sum << endl;
14 
15     return 0;
16 }

 1 #include <iostream>
 2 
 3 using namespace std;
 4 
 5 int main()
 6 {
 7     int num1 = 1, num2 = 2;
 8     int temp = 0;
 9     int sum = 0;
10     while (num2 < 4000001) {
11         if (num2 % 2 == 0) {
12             sum += num2;
13         }
14         temp = num1 + num2;
15         num1 = num2;
16         num2 = temp;
17     }
18     cout << sum << endl;
19 
20     return 0;
21 }

 1 #include <iostream>
 2 #include <cmath>
 3 
 4 using namespace std;
 5 
 6 bool isPrime(int);
 7 
 8 int main()
 9 {
10     double sq = sqrt(600851475143.0);
11     int num = sq;
12     for (int i = num; i > 1; -- i) {
13         if (600851475143 % i == 0 && isPrime(i)) {
14             cout << i << endl;
15             break;
16         }
17     }
18 
19     return 0;
20 }
21 
22 bool isPrime(int num)
23 {
24     if (num < 2) return false;
25     for (int i = 2; i <= sqrt((double)num); ++ i) {
26         if (num % i == 0) {
27             return false;
28         }
29     }
30     return true;
31 }