力扣23.合并K个升序链表

23.合并K个升序链表

思路1：

1. 将每个链表中所有元素全都放到新链表中
2. 用Collections工具给链表元素按大小排序
3. 新建ListNode串起来即可

代码实现

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists.length == 0) return null;
		int t;
		for (t = 0; t < lists.length; t++) {
			if (lists[t] != null) {
				break;
			}
		}
		if (t == lists.length) {
			return null;
		}
		List<Integer> list = new LinkedList<Integer>();
		for (int i = 0; i < lists.length; i++) {
			while (lists[i] != null) {
				list.add(lists[i].val);
				lists[i] = lists[i].next;
			}
		}
		Collections.sort(list, new Comparator<Integer>() {

			@Override
			public int compare(Integer o1, Integer o2) {
				// TODO Auto-generated method stub
				return o1 - o2;
			}
		});
		ListNode dummyHead = new ListNode(list.get(0));
        ListNode dummyHead1 = new ListNode(list.get(0));
		dummyHead1 = dummyHead;
		for (int i = 1; i < list.size(); i++) {
			dummyHead.next = new ListNode(list.get(i));
            dummyHead = dummyHead.next;
		}
		return dummyHead1;
    }
}

思路2:

1. 将每个链表所有元素放进优先级队列中，并自定义比较器
2. 创建ListNode串起来即可
3. 注意当优先级队列取完为空时，添加head.next = null

代码实现

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeKLists(ListNode[] lists) {
        
		PriorityQueue<ListNode> queue = new PriorityQueue<ListNode>(new Comparator<ListNode>() {

			@Override
			public int compare(ListNode o1, ListNode o2) {
				
				return o1.val - o2.val;
			}
		});
		for (ListNode head : lists) {
			while (head != null) {
				queue.offer(head);
				head = head.next;
			}
		}
		
		ListNode dummy = new ListNode(0);
		ListNode head = dummy;
		
		while (!queue.isEmpty()) {
			head.next = queue.poll();
			head = head.next;

            if (queue.isEmpty()) head.next = null; // 结束设置一个null
		}
		return dummy.next;
    }
}