LeetCode Hard: 23. Merge k Sorted Lists

一、题目

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

 合并K个有序数组

二、思路

递归或者分治法，先把K个数组分成2组，变成合并两组数组，接着一直分下去，递归到最后，变成合并两个单个数组

三、代码

#coding:utf-8
# Definition for singly-linked list.
import heapq
class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

class Solution0:
    def mergeKLists(self, lists):
        """
        :type lists: List[ListNode]
        :rtype: ListNode
        """
        """
        heapq使用说明
        a为普通列表 
        - heapq.heapify(a) 调整a，使得其满足最小堆 
        - heapq.heappop(a) 从最小堆中弹出最小的元素 
        - heapq.heappush(a,b) 向最小堆中压入新的元素
        heap = []
        for l in lists:
            if l != None:
                heap.append((l.val, l))
        heapq.heapify(heap)
        dummy = ListNode(0)
        cur = dummy
        while heap:
            _, h = heapq.heappop(heap)
            cur.next = h
            cur = cur.next
            if h.next:
                heapq.heappush(heap, (h.next.val, h.next))
        return dummy.next
        """
        if len(lists) == 0:
            return None
        if len(lists) == 1:
            return lists[0]
        #merge sort each two linked list
        l1 = self.mergeKLists(lists[:,len(lists)//2])
        l2 = self.mergeKLists(lists[len(lists)//2:])
        head = self.mergetwoLists(l1,l2)
        return head
    def mergetwoLists(self,l1,l2):
        if l1 is None:
            return l2
        if l2 is None:
            return l1
        p = ListNode(0)
        dummyhead = p
        while l1 is not None and l2 is not None:
            if l1.val < l2.val:
                p.next = l1
                l1 = l1.next
                p = p.next
            else:
                p.next = l2
                l2 = l2.next
                p = p.next
        if l1 is None:
            p.next = l2
        else:
            p.next = l1
        return dummyhead.next