leetcode Merge k Sorted Lists 难度系数3 3.11

yundomo

于 2014-02-01 10:46:02 发布

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本文提供了一种通过合并K个有序链表并返回单一有序链表的方法。使用递归方式将每个链表两两合并直至只剩下一个链表。文章包含完整的Java实现代码，并展示了如何进行两两链表合并。

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Question：

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

public class Solution {
	public ListNode mergeKLists(ArrayList<ListNode> lists) {
		if (lists.size() == 0) {
			return null;
		}
		if (lists.size() == 1) {
			return lists.get(0);
		}
		ListNode start = lists.get(0);
		for (int i = 1; i < lists.size(); i++) {
			start = mergeTwoLists(start, lists.get(i));
		}
		return start;
	}

	private ListNode mergeTwoLists(ListNode l1, ListNode l2) {
		ListNode p, q, r, head;
		if (l1 == null && l2 == null)
			return null;
		if (l1 == null)
			return l2;
		if (l2 == null)
			return l1;
		p = l1;
		q = l2;
		if (p.val <= q.val) {
			head = p;
			p = p.next;
		} else {
			head = q;
			q = q.next;
		}
		r = head;
		while (p != null && q != null) {
			if (p.val <= q.val) {
				r.next = p;
				r = p;
				p = p.next;
			} else {
				r.next = q;
				r = q;
				q = q.next;
			}
		}
		if (p == null) {
			r.next = q;
		} else {
			r.next = p;
		}
		return head;

	}
}