[leetcode]Merge k Sorted Lists @ Python

原題地址：https://oj.leetcode.com/problems/merge-k-sorted-lists/

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

解題思路：歸並k個已經排好序的鏈表。使用堆這一數據結構，首先將每條鏈表的頭節點進入堆中，然后將最小的彈出，並將最小的節點這條鏈表的下一個節點入堆，依次類推，最終形成的鏈表就是歸並好的鏈表。

代碼：

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

class Solution:
    # @param a list of ListNode
    # @return a ListNode
    def mergeKLists(self, lists):
        heap = []
        for node in lists:
            if node: 
                heap.append((node.val, node))
        heapq.heapify(heap)
        head = ListNode(0); curr = head
        while heap:
            pop = heapq.heappop(heap)
            curr.next = ListNode(pop[0])
            curr = curr.next
            if pop[1].next: 
                heapq.heappush(heap, (pop[1].next.val, pop[1].next))
        return head.next