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ReverseLinkedListInKgroup.java
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ReverseLinkedListInKgroup.java
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package Leetcode;
/**
* @author kalpak
*
* 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.
*
* Follow up:
*
* Could you solve the problem in O(1) extra memory space?
* You may not alter the values in the list's nodes, only nodes itself 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]
* 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
*/
public class ReverseLinkedListInKgroup {
public static ListNode reverseKGroup(ListNode head, int k) {
ListNode current = head;
int count = 0;
while(current != null && count != k) {
current = current.next;
count++;
}
if (count == k) {
ListNode prev = reverseKGroup(current, k); // Store the result of the recursive call as the previous node to be connected.
current = head; // recentre the current node.
while(count > 0) {
ListNode next = current.next;
current.next = prev;
prev = current;
current = next;
count --;
}
head = prev;
}
return head;
}
public static void main(String[] args) {
ListNode l1 = new ListNode(1);
l1.next = new ListNode(2);
l1.next.next = new ListNode(3);
l1.next.next.next = new ListNode(4);
l1.next.next.next.next = new ListNode(5);
ListNode result = reverseKGroup(l1, 2);
while(result != null) {
System.out.print(result.val + " ");
result = result.next;
}
}
}