https://leetcode-cn.com/problems/merge-k-sorted-lists/submissions/

小根堆

public static class ListNode {
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) { this.val = val; }
    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}
public ListNode mergeKLists(ListNode[] lists) {
    PriorityQueue<ListNode> heap = new PriorityQueue<>((ListNode l1, ListNode l2) -> {
        return l1.val - l2.val;
    });
    for (ListNode list : lists) {
        if (list != null) {
            heap.offer(list);
        }
    }
    ListNode head = new ListNode(0);
    ListNode cur = head;
    while (!heap.isEmpty()) {
        cur = cur.next = heap.poll();
        if (cur.next != null) {
            heap.offer(cur.next);
        }
    }
    cur.next = null;
    return head.next;
}