Leetcode•Jul 13, 2025
Reverse Nodes in k-Group
Hazrat Ali
Leetcode
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:
Input: head = [1,2,3,4,5], k = 2 Output: [2,1,4,3,5]
Example 2:
Input: head = [1,2,3,4,5], k = 3 Output: [3,2,1,4,5]
Solution
/**
* @param {ListNode} head
* @param {number} k
* @return {ListNode}
*/
const reverseKGroup = (head, k) => {
if (getLength(head) < k) {
return head;
}
let prev = null;
let curr = head;
let count = k;
while (curr && count-- > 0) {
let next = curr.next;
curr.next = prev;
prev = curr;
curr = next;
}
head.next = reverseKGroup(curr, k);
return prev;
};
const getLength = head => {
let count = 0;
let p = head;
while (p) {
p = p.next;
count++;
}
return count;
};