C++ Program For Reversing Alternate K Nodes In A Singly Linked List

Given a linked list, write a function to reverse every alternate k nodes (where k is an input to the function) in an efficient way. Give the complexity of your algorithm.

Example:

Inputs:   1->2->3->4->5->6->7->8->9->NULL and k = 3
Output:   3->2->1->4->5->6->9->8->7->NULL. 

Method 1 (Process 2k nodes and recursively call for rest of the list): 
This method is basically an extension of the method discussed in this post. 

kAltReverse(struct node *head, int k)
  1)  Reverse first k nodes.
  2)  In the modified list head points to the kth node.  So change next 
       of head to (k+1)th node
  3)  Move the current pointer to skip next k nodes.
  4)  Call the kAltReverse() recursively for rest of the n - 2k nodes.
  5)  Return new head of the list.

C++

// C++ program to reverse alternate
// k nodes in a linked list
#include <bits/stdc++.h>
using namespace std;

// Link list node
class Node
{
    public:
    int data;
    Node* next;
};

/* Reverses alternate k nodes and
   returns the pointer to the new
   head node */
Node *kAltReverse(Node *head, int k)
{
    Node* current = head;
    Node* next;
    Node* prev = NULL;
    int count = 0;

    /* 1) reverse first k nodes of the
       linked list */
    while (current != NULL && count < k)
    {
    next = current->next;
    current->next = prev;
    prev = current;
    current = next;
    count++;
    }

    /* 2) Now head points to the kth node.
       So change next  of head to (k+1)th node*/
    if(head != NULL)
    head->next = current;

    /* 3) We do not want to reverse next k
       nodes. So move the current
       pointer to skip next k nodes */
    count = 0;
    while(count < k-1 &&
          current != NULL )
    {
    current = current->next;
    count++;
    }

    /* 4) Recursively call for the list
       starting from current->next. And make
       rest of the list as next of first node */
    if(current != NULL)
    current->next = kAltReverse(current->next, k);

    /* 5) prev is new head of the input list */
    return prev;
}

// UTILITY FUNCTIONS
// Function to push a node
void push(Node** head_ref,
          int new_data)
{
    // Allocate node
    Node* new_node = new Node();

    //  Put in the data
    new_node->data = new_data;

    // Link the old list of the
    // new node
    new_node->next = (*head_ref);

    // Move the head to point to the
    // new node
    (*head_ref) = new_node;
}

// Function to print linked list
void printList(Node *node)
{
    int count = 0;
    while(node != NULL)
    {
        cout<<node->data<<" ";
        node = node->next;
        count++;
    }
}

// Driver code
int main(void)
{
    // Start with the empty list
    Node* head = NULL;
    int i;

    // Create a list
    // 1->2->3->4->5...... ->20
    for(i = 20; i > 0; i--)
    push(&head, i);

    cout << "Given linked list ";
    printList(head);
    head = kAltReverse(head, 3);

    cout << "Modified Linked list ";
    printList(head);
    return(0);
}
// This code is contributed by rathbhupendra

Output:

Given linked list
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Modified Linked list
3 2 1 4 5 6 9 8 7 10 11 12 15 14 13 16 17 18 20 19

Time Complexity: O(n)

Auxiliary Space: O(1)

Method 2 (Process k nodes and recursively call for rest of the list): 
The method 1 reverses the first k node and then moves the pointer to k nodes ahead. So method 1 uses two while loops and processes 2k nodes in one recursive call. 

This method processes only k nodes in a recursive call. It uses a third bool parameter b which decides whether to reverse the k elements or simply move the pointer.

_kAltReverse(struct node *head, int k, bool b)
  1)  If b is true, then reverse first k nodes.
  2)  If b is false, then move the pointer k nodes ahead.
  3)  Call the kAltReverse() recursively for rest of the n - k nodes and link 
       rest of the modified list with end of first k nodes. 
  4)  Return new head of the list.

C++

// C++ program to implement
// the above approach
#include <bits/stdc++.h>
using namespace std;

// Link list node
class node
{
    public:
    int data;
    node* next;
};

// Helper function for kAltReverse()
node * _kAltReverse(node *node,
                    int k, bool b);

// Alternatively reverses the given
// linked list in groups of given size k.
node *kAltReverse(node *head, int k)
{
    return _kAltReverse(head, k, true);
}

/* Helper function for kAltReverse().
   It reverses k nodes of the list only if
   the third parameter b is passed as true,
   otherwise moves the pointer k nodes ahead
   and recursively calls itself */
node * _kAltReverse(node *Node, int k, bool b)
{
    if(Node == NULL)
        return NULL;

    int count = 1;
    node *prev = NULL;
    node *current = Node;
    node *next;

    /* The loop serves two purposes
        1) If b is true,
           then it reverses the k nodes
        2) If b is false,
           then it moves the current pointer */
    while(current != NULL && count <= k)
    {
        next = current->next;

        // Reverse the nodes only if b is true
        if(b == true)
            current->next = prev;

        prev = current;
        current = next;
        count++;
    }

    /* 3) If b is true, then the node is the kth node.
        So attach the rest of the list after the node.
        4) After attaching, return the new head */
    if(b == true)
    {
        Node->next = _kAltReverse(current, k, !b);
        return prev;
    }

    /* If b is not true, then attach
       rest of the list after prev.
       So attach the rest of the list
       after prev */
    else
    {
        prev->next =
            _kAltReverse(current, k, !b);
        return Node;
    }
}

// UTILITY FUNCTIONS
// Function to push a node
void push(node** head_ref,
          int new_data)
{
    // Allocate node
    node* new_node = new node();

    // Put in the data
    new_node->data = new_data;

    // Link the old list of the
    // new node
    new_node->next = (*head_ref);

    // Move the head to point to
    // the new node
    (*head_ref) = new_node;
}

// Function to print linked list
void printList(node *node)
{
    int count = 0;
    while(node != NULL)
    {
        cout << node->data << " ";
        node = node->next;
        count++;
    }
}

// Driver Code
int main(void)
{
    // Start with the empty list
    node* head = NULL;
    int i;

    // Create a list
    // 1->2->3->4->5...... ->20
    for(i = 20; i > 0; i--)
    push(&head, i);

    cout << "Given linked list ";
    printList(head);
    head = kAltReverse(head, 3);

    cout << "Modified Linked list ";
    printList(head);
    return(0);
}
// This code is contributed by rathbhupendra

Output:

Given linked list
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Modified Linked list
3 2 1 4 5 6 9 8 7 10 11 12 15 14 13 16 17 18 20 19

Time Complexity: O(n) 

Auxiliary Space: O(n) for call stack because it is using recursion

Please refer complete article on Reverse alternate K nodes in a Singly Linked List for more details!