Python Program For Reversing Alternate K Nodes In A Singly Linked List
Last Updated : 22 Mar, 2023
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.
Python3 # Python3 program to reverse alternate # k nodes in a linked list import math # Link list node class Node: def __init__(self, data): self.data = data self.next = None # Reverses alternate k nodes and #returns the pointer to the new # head node def kAltReverse(head, k) : current = head next = None prev = None count = 0 # 1) reverse first k nodes of the # linked list while (current != None and count < k) : next = current.next current.next = prev prev = current current = next count = count + 1; # 2) Now head pos to the kth node. # So change next of head to (k+1)th node if(head != None): 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 and current != None ): current = current.next count = count + 1 # 4) Recursively call for the list # starting from current.next. And make # rest of the list as next of first node if(current != None): current.next = kAltReverse(current.next, k) # 5) prev is the new head of the # input list return prev # UTILITY FUNCTIONS # Function to push a node def push(head_ref, new_data): # Allocate node new_node = Node(new_data) # 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 return head_ref # Function to print linked list def printList(node): count = 0 while(node != None): print(node.data, end = " ") node = node.next count = count + 1 # Driver code if __name__=='__main__': # Start with the empty list head = None # Create a list # 1.2.3.4.5...... .20 for i in range(20, 0, -1): head = push(head, i) print("Given linked list ") printList(head) head = kAltReverse(head, 3) print("Modified Linked list") printList(head) # This code is contributed by Srathore 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) due to recursive stack space
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.
Python3 # Python code for above algorithm # Link list node class node: def __init__(self, data): self.data = data self.next = next # Function to insert a node at the # beginning of the linked list def push(head_ref, new_data): # Allocate node new_node = node(0) # Put in the data new_node.data = new_data # Link the old list to the # new node new_node.next = (head_ref) # Move the head to point to the # new node (head_ref) = new_node return head_ref """ Alternatively reverses the given linked list in groups of given size k. """ def kAltReverse(head, 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 call itself """ def _kAltReverse(Node, k, b) : if(Node == None) : return None count = 1 prev = None current = Node next = None """ 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 != None and count <= k) : next = current.next """ Reverse the nodes only if b is True """ if(b == True) : current.next = prev prev = current current = next count = count + 1 """ 3) If b is True, then node is the kth node. So attach rest of the list after node. 4) After attaching, return the new head """ if(b == True) : Node.next = _kAltReverse(current, k, not b) return prev else : """ If b is not True, then attach rest of the list after prev. So attach rest of the list after prev """ prev.next = _kAltReverse(current, k, not b) return Node """ Function to print linked list """ def printList(node) : count = 0 while(node != None) : print( node.data, end = " ") node = node.next count = count + 1 # Driver Code # Start with the empty list head = None i = 20 # Create a list # 1->2->3->4->5...... ->20 while(i > 0 ): head = push(head, i) i = i - 1 print("Given linked list ") printList(head) head = kAltReverse(head, 3) print("Modified Linked list ") printList(head) # This code is contributed by Arnab Kundu 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!
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