python 合并k个有序链表

 

from heapq import heappush, heappop


class Solution:
    def mergeKLists(self, lists):
        q = []
        for i,head in enumerate(lists):
            if head:
                heappush(q, (head.val, i, head))

        node = dummy = ListNode(0)
        while q:
            val, i, pop_node = heappop(q)
            print(val)
            node.next = ListNode(val)
            node = node.next
            next_node = pop_node.next
            if next_node:
                heappush(q, (next_node.val, i, next_node))
        return dummy.next

　　

 为啥有i？？？？？？？？？？？理由见后、

 

另外PQ也有这个问题

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


class Solution:
    def mergeKLists(self, lists: List[ListNode]) -> ListNode:
        q = queue.PriorityQueue()
        for i,head in enumerate(lists):
            if head:
                q.put((head.val, i, head))
        
        node = dummy = ListNode(0)
        while not q.empty():
            val, i, pop_node = q.get()
            node.next = ListNode(val)
            node = node.next
            next_node = pop_node.next
            if next_node:
                q.put((next_node.val, i, next_node))
        return dummy.next

　　

python 3 heappush Line 13: TypeError: '<' not supported between instances of 'ListNode' and 'ListNode'

出现原因：

@cbmbbz In the event that two or more of the lists have the same val, this code will error out since the queue module will compare the second element in the priority queue which is a ListNode object (and this is not a comparable type).

 

To solve for this issue, I have stored (node.val, list_idx, node) to account for this edge case.

 

from queue import PriorityQueue
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def mergeKLists(self, lists):
        k = len(lists)
        q = PriorityQueue(maxsize=k)
        dummy = ListNode(None)
        curr = dummy
        for list_idx, node in enumerate(lists):
            if node: q.put((node.val, list_idx, node))
        while q.qsize() > 0:
            poped = q.get()
            curr.next, list_idx = poped[2], poped[1]
            curr = curr.next
            if curr.next: q.put((curr.next.val, list_idx, curr.next))
        return dummy.next

 

 

 

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onedingz25

Last Edit: October 11, 2018 9:42 PM

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Python3 env will get TypeError: '<' not supported between instances of 'ListNode' and 'ListNode'
if there are duplicate values in the list. Works in Python env.