[LintCode 104] 合并k个排序链表(Python)

题目描述

合并k个排序链表，并且返回合并后的排序链表。尝试分析和描述其复杂度。
样例
给出3个排序链表[2->4->null,null,-1->null]，返回 -1->2->4->null

思路

两两合并，如果是奇数，余出来的直接加在最后。

代码

"""
Definition of ListNode
class ListNode(object):

    def __init__(self, val, next=None):
        self.val = val
        self.next = next
"""


class Solution:
    """
    @param lists: a list of ListNode
    @return: The head of one sorted list.
    """
    def mergeKLists(self, lists):
        # write your code here
        if lists is None or len(lists) == 0:
            return None
        while len(lists) > 1:
            tmp = []
            for i in range(0, len(lists) - 1, 2):
                p = self.mergeTwoList(lists[i], lists[i + 1])
                tmp.append(p)
            if len(lists) % 2 == 1:
                tmp.append(lists[-1])
            lists = tmp
        return lists[0]

    def mergeTwoList(self, l1, l2):
        if l1 is None:
            return l2
        if l2 is None:
            return l1
        res = ListNode(0)
        r = res
        while l1 is not None and l2 is not None:
            if l1.val <= l2.val:
                r.next = l1
                l1 = l1.next
            else:
                r.next = l2
                l2 = l2.next
            r = r.next
        if l1 is not None:
            r.next = l1
        if l2 is not None:
            r.next = l2
        return res.next

复杂度分析

时间复杂度$O(nklogk)$，空间复杂度$O(1)$。