Merge k Sorted Lists

/*
 * Problem : Process lines.
 * Input   : Several lines.
 * Output  : The merged list.
 * Approach: Heap.
 * Time    : O(N log k)
 * Space   : O(k)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static String solve(String[] lines) {
        // Step 1: Read the lists. The heap holds {value, list number, position}. It always gives the smallest value of the front numbers.
        int k = lines.length;
        long[][] lists = new long[k][];
        java.util.PriorityQueue<long[]> heap = new java.util.PriorityQueue<long[]>((x, y) -> Long.compare(x[0], y[0]));
        for (int i = 0; i < k; i++) {
            String[] part = lines[i].split(" ");
            lists[i] = new long[part.length];
            for (int j = 0; j < part.length; j++) {
                lists[i][j] = Long.parseLong(part[j]);
            }
            heap.add(new long[] {lists[i][0], i, 0});
        }
        // Step 2: Take the smallest, write it, and push the next number of the same list.
        StringBuilder result = new StringBuilder();
        while (!heap.isEmpty()) {
            long[] top = heap.poll();
            result.append(result.length() == 0 ? "" : " ").append(top[0]);
            int list = (int) top[1];
            int next = (int) top[2] + 1;
            if (next < lists[list].length) {
                heap.add(new long[] {lists[list][next], list, next});
            }
        }
        return result.toString();
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        java.util.List<String> all = new java.util.ArrayList<>();
        while (scanner.hasNextLine()) {
            String line = scanner.nextLine().trim();
            if (!line.isEmpty()) {
                all.add(line);
            }
        }
        String[] lines = all.toArray(new String[0]);
        System.out.println(solve(lines));
    }
}