23. Merge k Sorted Lists(Hard)

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Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

Example:

Input:
[
1->4->5,
1->3->4,
2->6
]
Output: 1->1->2->3->4->4->5->6

优先队列

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists == null || lists.length == 0) return null;
        
        PriorityQueue<ListNode> p = new PriorityQueue<>(new Comparator<ListNode>(){//复习时候多看看，不熟练
            @Override
            public int compare(ListNode n1, ListNode n2){
                if (n1.val < n2.val) return -1;
                else if (n1.val == n2.val) return 0;
                else return 1;
            }
        });
            
        ListNode ans = new ListNode(0), tail = ans;
        
        for (ListNode o : lists)
            if (o != null) p.offer(o);
        
        while (!p.isEmpty()){
            ListNode t = p.poll();
            tail.next = t;
            tail = tail.next;
            if (t.next != null) p.offer(t.next);
        }
            
         return ans.next;   
            
    }
}