Java [leetcode 23]Merge k Sorted Lists

题目描述：

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

解题思路：

分治方法。将K个List不断地分解为前半部分和后半部分。分别进行两个List的合并。最后将合并的结果合并起来。

代码如下：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {

    public ListNode mergeKLists(ListNode[] lists) {
		if(lists.length == 0)
			return null;
		return divide(lists, 0, lists.length - 1);
	}

	public ListNode divide(ListNode[] l1, int left, int right) {
		if(left < right){
			int mid = (left + right) / 2;
			return mergeTwoLists(divide(l1, left, mid), divide(l1, mid + 1, right));
		}
		return l1[left];
	}

	public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
		ListNode list = new ListNode(0);
		ListNode tmp = list;
		while (l1 != null || l2 != null) {
			if (l1 == null) {
				tmp.next = new ListNode(l2.val);
				l2 = l2.next;
			} else if (l2 == null) {
				tmp.next = new ListNode(l1.val);
				l1 = l1.next;
			} else {
				if (l1.val < l2.val) {
					tmp.next = new ListNode(l1.val);
					l1 = l1.next;
				} else {
					tmp.next = new ListNode(l2.val);
					l2 = l2.next;
				}
			}
			tmp = tmp.next;
		}
		return list.next;
	}
}