LeetCode : Merge k Sorted Lists [java]

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#LeetCode #Merge k Sorted Lists

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本文介绍了一种使用归并法来合并K个有序链表的方法，并提供了详细的Java实现代码。该方法通过递归地将链表分成两部分，分别进行合并后再合并两个链表，实现了高效地合并K个有序链表。

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

思路：归并法，主要利用切分简化问题。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
 
import java.util.Arrays;
 
public class Solution {
	public ListNode mergeKLists(ListNode[] lists) {
		if (lists == null || lists.length == 0) {
			return null;
		}
		if (lists.length == 1) {
			return lists[0];
		}

		int mid = lists.length / 2;
		ListNode[] firstList = Arrays.copyOfRange(lists, 0, mid);
		ListNode[] lastList = Arrays.copyOfRange(lists, mid, lists.length);
		ListNode l1 = mergeKLists(firstList);
		ListNode l2 = mergeKLists(lastList);
		return merge2List(l1, l2);
	}

	private ListNode merge2List(ListNode l1, ListNode l2) {
		ListNode head = new ListNode(-1);
		ListNode p = head;
		if (l1 == null) {
			return l2;
		}
		if (l2 == null) {
			return l1;
		}
		while (l1 != null && l2 != null) {
			if (l1.val < l2.val) {
				p.next = l1;
				p = p.next;
				l1 = l1.next;
			} else {
				p.next = l2;
				p = p.next;
				l2 = l2.next;
			}
		}
		if (l1 != null) {
			p.next = l1;
		}
		if (l2 != null) {
			p.next = l2;
		}
		return head.next;
	}
}