【LeetCode】23. Merge k Sorted Lists - Java实现

1. 题目描述：

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

2. 思路分析：

题目的意思是合并k个有序的链表。

这题可以运用分治法，进行两两合并，转化成第21题(Merge Two Sorted Lists)去解决，知道最后只剩下一个链表。如下图所示：

3. Java代码：

源代码：见我GiHub主页

代码：

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

    int length = lists.length;
    int interval = 1;
    while (interval < length) {
        for (int i = 0; i < length - interval; i += 2 * interval) {
            lists[i] = merge2Lists(lists[i], lists[i + interval]);
        }
        interval *= 2;
    }
    return lists[0];
}

private static ListNode merge2Lists(ListNode list1, ListNode list2) {
    ListNode head = new ListNode(0);
    ListNode cur = head;
    while (list1 != null && list2 != null) {
        if (list1.val < list2.val) {
            cur.next = list1;
            list1 = list1.next;
        } else {
            cur.next = list2;
            list2 = list2.next;
        }
        cur = cur.next;
    }

    if (list1 != null) {
        cur.next = list1;
    }

    if (list2 != null) {
        cur.next = list2;
    }

    return head.next;
}