题解

#include <bits/stdc++.h>
using namespace std;

bool isPrime(int num) {
    if (num < 2) {
        return false;
    }
    for (int i = 2; i * i <= num; i++) {
        if (num % i == 0) {
            return false;
        }
    }
    return true;
}

int main() {
    int N;
    cin >> N;

    vector<bool> isCom(N + 1, false);
    vector<int> primes;

    for (int i = 2; i <= N; i++) {
        if (!isCom[i]) {
            primes.push_back(i);
            for (int j = i * 2; j <= N; j += i) {
                isCom[j] = true;
            }
        }
    }

    for (int num = 4; num <= N; num += 2) {
        bool found = false;
        int prime1 = 0, prime2 = 0;

        for (int i = 0; i <= primes.size() && primes[i] <= num / 2; i++) {
            int complement = num - primes[i];
            if (isPrime(complement)) {
                found = true;
                prime1 = primes[i];
                prime2 = complement;
                break;
            }
        }

        if (!found) {
            cout << num << " cannot be expressed as the sum of two prime numbers." << endl;
        } else {
            cout << num << "=" << prime1 << "+" << prime2 << endl;
        }
    }

    return 0;
}

太经典了