Reverse A Linked List In Groups Of K Problem Statement
Given a linked list, reverse every group of k nodes. If there is a remainder (a group of less than k nodes) in the end, reverse that last group, too.
Example One
{
"head": [1, 2, 3, 4, 5, 6],
"k": 3
}
Output:
[3, 2, 1, 6, 5, 4]
Input list consists of two whole groups of three. In the output list the first three and last three nodes are reversed.
Example Two
{
"head": [1, 2, 3, 4, 5, 6, 7, 8],
"k": 3
}
Output:
[3, 2, 1, 6, 5, 4, 8, 7]
There are two whole groups of three and one partial group (a remainder that consists of just two nodes). Each of the three groups is reversed in the output.
Notes
- The function has two parameters: head of the given linked list and
k.
- Return the head of the linked list after reversing the groups of nodes in it.
Constraints:
- 1 <= number of nodes in the list <= 100000
- -2 * 109 <= node value <= 2 * 109
- 1 <=
k <= number of nodes
- Cannot use more than constant extra space
We have provided one solution for this problem.
Reverse A Linked List In Groups Of K Solution: Iterative
The "constant extra space" constraint rules out using recursion because recursion uses the call stack space (up to O(n) if k = n).
We are forced to use an iterative approach.
This solution demonstrates how that can be implemented.
Time Complexity
O(n).
Auxiliary Space Used
O(1).
Space Complexity
O(n).
Code For Reverse A Linked List In Groups Of K Solution: Iterative
/*
Asymptotic complexity in terms of length of given linked list \`n\`:
* Time: O(n).
* Auxiliary space: O(1).
* Total space: O(n).
*/
// Helper function: Reverse singly linked list in O(length) time and O(1) extra space.
void reverse_linked_list(LinkedListNode *cur)
{
LinkedListNode *prev = NULL;
LinkedListNode *next;
while (cur)
{
next = cur->next;
cur->next = prev;
prev = cur;
cur = next;
}
}
LinkedListNode *reverse_linked_list_in_groups_of_k(LinkedListNode *head, int k)
{
/*
Input:
list: 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> NULL
k: 3
Output:
3 -> 2 -> 1 -> 6 -> 5 -> 4 -> 8 -> 7-> NULL
Groups to be reversed are (1 -> 2 -> 3), (4 -> 5 -> 6) and (7 -> 8).
We will call reverse_linked_list function when (start = 1 and stop = 3),
(start = 4 and stop = 6) and (start = 7 and stop = 8).
*/
// Points to previous start node.
LinkedListNode *prev_of_start = NULL;
LinkedListNode *start = head;
LinkedListNode *stop = head;
int count = 0;
// Traverse whole linked list.
while (stop)
{
count++;
/*
If we have covered k nodes in between start
and stop (inclusive) or we are at the last node.
*/
if (count == k || stop->next == NULL)
{
// Points to next node of start.
LinkedListNode *next_of_stop = stop->next;
/*
We want to reverse start to stop nodes,
set stop->next = NULL so we know where to stop.
*/
stop->next = NULL;
// Reverse start to stop nodes.
reverse_linked_list(start);
if (prev_of_start == NULL)
{
// Head will change when we are reversing the linked list first time.
head = stop;
}
else
{
/*
We have reversed start to stop nodes, hence now stop will be next node of
prev_of_start.
*/
prev_of_start->next = stop;
}
/*
We have reversed start to stop nodes, hence next_of_stop will be next node of start.
*/
start->next = next_of_stop;
/*
In the above example, after we have reversed first k nodes list will be:
3 -> 2 -> 1 -> 4 -> 5 -> 6 -> 7 -> 8 -> NULL,
start will point to 1, next_of_stop will point to 4.
Now we will set start and stop to point at 4.
And prev_of_start should be previous of 4 that is 1.
*/
prev_of_start = start;
start = next_of_stop;
stop = next_of_stop;
// Reset counter.
count = 0;
}
else
{
stop = stop->next;
}
}
return head;
}
We hope that these solutions to reverse a linked list in groups of k problem have helped you level up your coding skills. You can expect problems like these at top tech companies like Amazon and Google.
If you are preparing for a tech interview at FAANG or any other Tier-1 tech company, register for Interview Kickstart's FREE webinar to understand the best way to prepare.
Interview Kickstart offers interview preparation courses taught by FAANG+ tech leads and seasoned hiring managers. Our programs include a comprehensive curriculum, unmatched teaching methods, and career coaching to help you nail your next tech interview.
We offer 18 interview preparation courses, each tailored to a specific engineering domain or role, including the most in-demand and highest-paying domains and roles, such as:
To learn more, register for the FREE webinar.
.png)
