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花花酱 LeetCode 92. Reverse Linked List II

Reverse a linked list from position m to n. Do it in one-pass.

Note: 1 ≤ m ≤ n ≤ length of list.

Example:

Input: 1->2->3->4->5->NULL, m = 2, n = 4
Output: 1->4->3->2->5->NULL

Solution:

  1. Store the m – 1 and m-th item as prev and tail before reversing
  2. Reverse the m to n, return the head and tail->next of the reversed list
  3. Reconnect prev and head, tail and tail->next

Time complexity: O(n)
Space complexity: O(1)

C++

Python3

花花酱 LeetCode 23. Merge k Sorted Lists

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

Solution 1: Min heap

Time complexity: O(nklogk)
Space complexity: O(k)

C++

Solution 2: Merge Sort

Time complexity: O(nklogk)
Space complexity: O(logk)

C++

花花酱 LeetCode 25. Reverse Nodes in k-Group

Problem

Given a linked list, reverse the nodes of a linked list k at a time and return its modified list.

k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is.

Example:

Given this linked list: 1->2->3->4->5

For k = 2, you should return: 2->1->4->3->5

For k = 3, you should return: 3->2->1->4->5

Note:

  • Only constant extra memory is allowed.
  • You may not alter the values in the list’s nodes, only nodes itself may be changed.

Solution

Two passes.

First pass, get the length of the list.

Second pass, swap in groups.

Time complexity: O(n)

Space complexity: O(1)

C++

Related Problems

花花酱 LeetCode 24. Swap Nodes in Pairs

Problem

Given a linked list, swap every two adjacent nodes and return its head.

Example:

Given 1->2->3->4, you should return the list as 2->1->4->3.

Note:

  • Your algorithm should use only constant extra space.
  • You may not modify the values in the list’s nodes, only nodes itself may be changed.

Solution

Time complexity: O(n)

Space complexity: O(1)

C++

Python3

Related Problems

花花酱 LeetCode 23. Merge k Sorted Lists

Problem

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

Solution 1: Brute Force

Time complexity: O(nk)

Space complexity: O(1)

C++

Solution 2: Heap / Priority Queue

Time complexity: O(nlogk)

Space complexity: O(k)

C++