AOJ 0056 Goldbach's Conjecture
概要
二つの素数の和がnとなる組み合わせの数を求めよ
コード
#include <iostream> #include <cstdio> #include <vector> using namespace std; vector<int> sieveOfEratosthenes(int n) { vector<int> arr(n + 1); vector<int> prime; for (int i = 0; i <= n; i++) arr[i] = 1; arr[0] = arr[1] = 0; for (int i = 2; i*i < n; i++) if (arr[i]) for (int j = i * i; j < n; j += i) arr[j] = 0; for (int i = 2; i <= n; i++) { if (arr[i] == 1) prime.push_back(i); } return prime; } int main() { vector<int> prime = sieveOfEratosthenes(60000); int ans[50001] = {0}; int n; int size = prime.size(); for (int i = 0; i < size; i++) { for (int j = i; j < size; j++) { if (prime[i] + prime[j] > 50000) continue; ans[prime[i] + prime[j]]++; } } while (1) { cin >> n; if (n == 0) break; cout << ans[n] << endl; } return 0; }