LeetCode 2761. 和等于目标值的质数对

对于这题，我们可以利用素数筛将<n的素数筛出，随后同两数之和的做法。

class Solution {
public:
    vector<vector<int>> findPrimePairs(int n) {
        vector<vector<int>> ans;
        if (n < 4) return ans;
        vector<bool> prime(n + 1, true);
        prime[0] = prime[1] = false;
        for (int i = 2; 1LL * i * i <= n; ++i) {
            if (prime[i]) {
                for (int j = i * i; j <= n; j += i) {
                    prime[j] = false;
                }
            }
        }
        for (int x = 2; x <= n / 2; ++x) {
            int y = n - x;
            if (prime[x] && prime[y]) {
                ans.push_back({x, y});
            }
        }
        return ans;
    }
};