Solution.cs

namespace LeetCodeNet.G0001_0100.S0025_reverse_nodes_in_k_group {

// #Hard #Top_100_Liked_Questions #Linked_List #Recursion #Data_Structure_II_Day_13_Linked_List
// #Udemy_Linked_List #Big_O_Time_O(n)_Space_O(k)
// #2023_12_26_Time_75_ms_(86.97%)_Space_43.9_MB_(5.81%)

using LeetCodeNet.Com_github_leetcode;

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int val=0, ListNode next=null) {
 *         this.val = val;
 *         this.next = next;
 *     }
 * }
 */
public class Solution {
    public ListNode ReverseKGroup(ListNode head, int k) {
        if (head == null || head.next == null || k == 1) {
            return head;
        }
        int j = 0;
        ListNode len = head;
        // loop for checking the length of the linked list, if the linked list is less than k, then return
        // as it is.
        while (j < k) {
            if (len == null) {
                return head;
            }
            len = len.next;
            j++;
        }
        // Reverse linked list logic applied here.
        ListNode c = head;
        ListNode n = null;
        ListNode prev = null;
        int i = 0;
        // Traverse the while loop for K times to reverse the nodes in K groups.
        while (i != k) {
            n = c.next;
            c.next = prev;
            prev = c;
            c = n;
            i++;
        }
        // `head` is pointing to the 1st node of the K group, which is now going to point to the next K group
        // linked list.
        // Recursion for further remaining linked list.
        head.next = ReverseKGroup(n, k);
        return prev;
    }
}
}