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Java-based LeetCode algorithm problem solutions, regularly updated

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package g2701_2800.s2761_prime_pairs_with_target_sum;

// #Medium #Array #Math #Enumeration #Number_Theory
// #2023_09_25_Time_57_ms_(99.24%)_Space_56.4_MB_(10.58%)

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;

/**
 * 2761 - Prime Pairs With Target Sum\.
 *
 * Medium
 *
 * You are given an integer `n`. We say that two integers `x` and `y` form a prime number pair if:
 *
 * *   `1 <= x <= y <= n`
 * *   `x + y == n`
 * *   `x` and `y` are prime numbers
 *
 * Return _the 2D sorted list of prime number pairs_ [x_i, y_i]. The list should be sorted in **increasing** order of x_i. If there are no prime number pairs at all, return _an empty array_.
 *
 * **Note:** A prime number is a natural number greater than `1` with only two factors, itself and `1`.
 *
 * **Example 1:**
 *
 * **Input:** n = 10
 *
 * **Output:** [[3,7],[5,5]]
 *
 * **Explanation:**
 *
 * In this example, there are two prime pairs that satisfy the criteria.
 *
 * These pairs are [3,7] and [5,5], and we return them in the sorted order as described in the problem statement.
 *
 * **Example 2:**
 *
 * **Input:** n = 2
 *
 * **Output:** []
 *
 * **Explanation:**
 *
 * We can show that there is no prime number pair that gives a sum of 2, so we return an empty array.
 *
 * **Constraints:**
 *
 * *   1 <= n <= 10⁶
**/
public class Solution {
    private static final HashSet PRIMES = new HashSet<>();
    private static final List LIST = new ArrayList<>();

    public List> findPrimePairs(int n) {
        List> answer = new ArrayList<>();
        for (int a : LIST) {
            int other = n - a;
            if (other < n / 2 || a > n / 2) {
                break;
            }
            if (PRIMES.contains(other)) {
                answer.add(List.of(a, other));
            }
        }
        return answer;
    }

    static {
        int m = 1000001;
        boolean[] visited = new boolean[m];
        for (int i = 2; i < m; i++) {
            if (!visited[i]) {
                PRIMES.add(i);
                LIST.add(i);
                int j = i;
                while (j < m) {
                    visited[j] = true;
                    j += i;
                }
            }
        }
    }
}