23. Merge k Sorted Lists

Merge k sorted linked lists and return it as one sorted list.

Analyze and describe its complexity.

Example

Given lists:

[
  2->4->null,
  null,
  -1->null
],

return -1->2->4->null.

分析：

利用heap来保存数组中每一个list的第一个node, 当取出一个node以后，再把那个node的next node加进来，直到所有node都已被加进来。

class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        Queue<ListNode> heap = new PriorityQueue<ListNode>((n1, n2) -> n1.val - n2.val);
        Arrays.stream(lists).filter(item -> item != null).forEach(item -> heap.add(item));

        ListNode head = new ListNode(-1);
        ListNode cur = head;
        while (heap.size() > 0) {
            ListNode node = heap.poll();
            cur.next = node;
            cur = node;
            if (node.next != null) {
                heap.add(node.next);
            }
        }
        return head.next;
    }
}