g0001_0100.s0025_reverse_nodes_in_k_group.readme.md Maven / Gradle / Ivy

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25\. Reverse Nodes in k-Group

Hard

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.

You may not alter the values in the list's nodes, only nodes themselves may be changed.

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/10/03/reverse_ex1.jpg)

**Input:** head = \[1,2,3,4,5\], k = 2

**Output:** \[2,1,4,3,5\] 

**Example 2:**

![](https://assets.leetcode.com/uploads/2020/10/03/reverse_ex2.jpg)

**Input:** head = \[1,2,3,4,5\], k = 3

**Output:** \[3,2,1,4,5\] 

**Example 3:**

**Input:** head = \[1,2,3,4,5\], k = 1

**Output:** \[1,2,3,4,5\] 

**Example 4:**

**Input:** head = \[1\], k = 1

**Output:** \[1\] 

**Constraints:**

*   The number of nodes in the list is in the range `sz`.
*   `1 <= sz <= 5000`
*   `0 <= Node.val <= 1000`
*   `1 <= k <= sz`

**Follow-up:** Can you solve the problem in O(1) extra memory space?