Given a singly linked list, write a function to swap elements pairwise. For example, if the linked list is 1->2->3->4->5->6->7 then the function should change it to 2->1->4->3->6->5->7, and if the linked list is 1->2->3->4->5->6 then the function should change it to 2->1->4->3->6->5
This problem has been discussed here. The solution provided there swaps data of nodes. If data contains many fields, there will be many swap operations. So changing links is a better idea in general. Following is the implementation that changes links instead of swapping data.
# Python3 program to swap elements of # linked list by changing links # Linked List Node class Node:
def __init__( self , data):
self .data = data
self . next = None
# Create and Handle list # operations class LinkedList:
def __init__( self ):
# Head of list
self .head = None
# Add data to list
def addToList( self , data):
newNode = Node(data)
if self .head is None :
self .head = newNode
return
last = self .head
while last. next :
last = last. next
last. next = newNode
# Function to print nodes
# in a given linked list
def __str__( self ):
linkedListStr = ""
temp = self .head
while temp:
linkedListStr = (linkedListStr +
str (temp.data) + " " )
temp = temp. next
return linkedListStr
# Function to pairwise swap elements
# of a linked list. It returns head of
# the modified list, so return value
# of this node must be assigned
def pairWiseSwap( self ):
# If list is empty or with one
# node
if ( self .head is None or
self .head. next is None ):
return
# Initialize previous and current
# pointers
prevNode = self .head
currNode = self .head. next
# Change head node
self .head = currNode
# Traverse the list
while True :
nextNode = currNode. next
# Change next of current
# node to previous node
currNode. next = prevNode
# If next node is the last node
if nextNode. next is None :
prevNode. next = nextNode
break
# Change next of previous to
# next of next
prevNode. next = nextNode. next
# Update previous and current nodes
prevNode = nextNode
currNode = prevNode. next
# Driver Code linkedList = LinkedList()
linkedList.addToList( 1 )
linkedList.addToList( 2 )
linkedList.addToList( 3 )
linkedList.addToList( 4 )
linkedList.addToList( 5 )
linkedList.addToList( 6 )
linkedList.addToList( 7 )
print ( "Linked list before calling" "pairwiseSwap() " , linkedList)
linkedList.pairWiseSwap() print ( "Linked list after calling "
"pairwiseSwap() " , linkedList)
# This code is contributed by AmiyaRanjanRout |
Output:
Linked list before calling pairWiseSwap() 1 2 3 4 5 6 7 Linked list after calling pairWiseSwap() 2 1 4 3 6 5 7
Time Complexity: The time complexity of the above program is O(n) where n is the number of nodes in a given linked list. The while loop does a traversal of the given linked list.
Auxiliary Space: O(1) because constant space has been used
Following is the recursive implementation of the same approach. We change the first two nodes and recur for the remaining list. Thanks to geek and omer salem for suggesting this method.
# Python3 program to pairwise swap # linked list using recursive method # Linked List Node class Node:
def __init__( self , data):
self .data = data
self . next = None
# Create and Handle list # operations class LinkedList:
def __init__( self ):
# Head of list
self .head = None # Add data to list
def addToList( self , data):
newNode = Node(data)
if self .head is None :
self .head = newNode
return
last = self .head
while last. next :
last = last. next
last. next = newNode
# Function to print nodes in
# a given linked list
def __str__( self ):
linkedListStr = ""
temp = self .head
while temp:
linkedListStr = (linkedListStr +
str (temp.data) + " " )
temp = temp. next
return linkedListStr
# Function to pairwise swap elements of
# a linked list. It returns head of the
# modified list, so return value
# of this node must be assigned
def pairWiseSwap( self , node):
# If list is empty or with one node
if node is None or node. next is None :
return node
# Store head of list after
# 2 nodes
remaining = node. next . next
# Change head
newHead = node. next
# Change next to second node
node. next . next = node
# Recur for remaining list and
# change next of head
node. next = self .pairWiseSwap(remaining)
# Return new head of modified list
return newHead
# Driver Code linkedList = LinkedList()
linkedList.addToList( 1 )
linkedList.addToList( 2 )
linkedList.addToList( 3 )
linkedList.addToList( 4 )
linkedList.addToList( 5 )
linkedList.addToList( 6 )
linkedList.addToList( 7 )
print ( "Linked list before calling "
"pairwiseSwap() " , linkedList)
linkedList.head = linkedList.pairWiseSwap(
linkedList.head)
print ( "Linked list after calling "
"pairwiseSwap() " , linkedList)
# This code is contributed by AmiyaRanjanRout |
Output:
Linked list before calling pairWiseSwap() 1 2 3 4 5 6 7 Linked list after calling pairWiseSwap() 2 1 4 3 6 5 7
Please refer complete article on Pairwise swap elements of a given linked list by changing links for more details!