23.Merge k Sorted Lists

tags: Hard,Linked List,Heap,Divide and Conquer

23. Merge k Sorted Lists

題目描述

You are given an array of k linked-lists lists, each linked-list is sorted in ascending order.

Merge all the linked-lists into one sorted linked-list and return it.

範例

Example 1:

Input: lists = [[1,4,5],[1,3,4],[2,6]]
Output: [1,1,2,3,4,4,5,6]
Explanation: The linked-lists are:
[
  1->4->5,
  1->3->4,
  2->6
]
merging them into one sorted list:
1->1->2->3->4->4->5->6

Example 2:

Input: lists = []
Output: []

Example 3:

Input: lists = [[]]
Output: []

Constraints:

• k == lists.length
• 0 <= k <= 10⁴
• 0 <= lists[i].length <= 500
• -10⁴ <= lists[i][j] <= 10⁴
• lists[i] is sorted in ascending order.
• The sum of lists[i].length will not exceed 10⁴.

解答

Python

class Solution:
    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
        def merge2Lists(l1, l2):
            head = ListNode()
            cur = head
            while l1 and l2:
                if l1.val <= l2.val:
                    cur.next = l1
                    l1 = l1.next
                else:
                    cur.next = l2
                    l2 = l2.next
                cur = cur.next
            if l1: cur.next = l1
            else: cur.next = l2
            return head.next
        
        N = len(lists)
        if N <= 0: return None
        step = 1
        while step < N:
            for i in range(0, N - step, step*2):
                lists[i] = merge2Lists(lists[i], lists[i + step])
            step *= 2
        return lists[0]

Yen-Chi ChenSun, Mar 12, 2023

Reference

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