Given a Singly Linked List, starting from the second node delete all alternate nodes of it. For example, if the given linked list is 1->2->3->4->5 then your function should convert it to 1->3->5, and if the given linked list is 1->2->3->4 then convert it to 1->3.
Method 1 (Iterative) Keep track of previous of the node to be deleted. First, change the next link of the previous node and iteratively move to the next node.
C++
// C++ program to remove alternate
// nodes of a linked list
#include <bits/stdc++.h>
usingnamespacestd;
/* A linked list node */
classNode
{
public:
intdata;
Node *next;
};
/* deletes alternate nodes
of a list starting with head */
voiddeleteAlt(Node *head)
{
if(head == NULL)
return;
/* Initialize prev and node to be deleted */
Node *prev = head;
Node *node = head->next;
while(prev != NULL && node != NULL)
{
/* Change next link of previous node */
prev->next = node->next;
delete(node); // delete the node
/* Update prev and node */
prev = prev->next;
if(prev != NULL)
node = prev->next;
}
}
/* UTILITY FUNCTIONS TO TEST fun1() and fun2() */
/* Given a reference (pointer to pointer) to the head
of a list and an int, push a new node on the front
of the list. */
voidpush(Node** head_ref, intnew_data)
{
/* allocate node */
Node* new_node = newNode();
/* put in the data */
new_node->data = new_data;
/* link the old list off the new node */
new_node->next = (*head_ref);
/* move the head to point to the new node */
(*head_ref) = new_node;
}
/* Function to print nodes in a given linked list */
voidprintList(Node *node)
{
while(node != NULL)
{
cout<< node->data<<" ";
node = node->next;
}
}
/* Driver code */
intmain()
{
/* Start with the empty list */
Node* head = NULL;
/* Using push() to construct below list
1->2->3->4->5 */
push(&head, 5);
push(&head, 4);
push(&head, 3);
push(&head, 2);
push(&head, 1);
cout<<"List before calling deleteAlt() \n";
printList(head);
deleteAlt(head);
cout<<"\nList after calling deleteAlt() \n";
printList(head);
return0;
}
// This code is contributed by rathbhupendra
C
// C program to remove alternate nodes of a linked list
#include<stdio.h>
#include<stdlib.h>
/* A linked list node */
structNode
{
intdata;
structNode *next;
};
/* deletes alternate nodes of a list starting with head */
voiddeleteAlt(structNode *head)
{
if(head == NULL)
return;
/* Initialize prev and node to be deleted */
structNode *prev = head;
structNode *node = head->next;
while(prev != NULL && node != NULL)
{
/* Change next link of previous node */
prev->next = node->next;
/* Free memory */
free(node);
/* Update prev and node */
prev = prev->next;
if(prev != NULL)
node = prev->next;
}
}
/* UTILITY FUNCTIONS TO TEST fun1() and fun2() */
/* Given a reference (pointer to pointer) to the head
of a list and an int, push a new node on the front
of the list. */
voidpush(structNode** head_ref, intnew_data)
{
/* allocate node */
structNode* new_node =
(structNode*) malloc(sizeof(structNode));
/* put in the data */
new_node->data = new_data;
/* link the old list off the new node */
new_node->next = (*head_ref);
/* move the head to point to the new node */
(*head_ref) = new_node;
}
/* Function to print nodes in a given linked list */
voidprintList(structNode *node)
{
while(node != NULL)
{
printf("%d ", node->data);
node = node->next;
}
}
/* Driver program to test above functions */
intmain()
{
/* Start with the empty list */
structNode* head = NULL;
/* Using push() to construct below list
1->2->3->4->5 */
push(&head, 5);
push(&head, 4);
push(&head, 3);
push(&head, 2);
push(&head, 1);
printf("\nList before calling deleteAlt() \n");
printList(head);
deleteAlt(head);
printf("\nList after calling deleteAlt() \n");
printList(head);
return0;
}
Java
// Java program to delete alternate nodes of a linked list
classLinkedList {
Node head; // head of list
/* Linked list Node*/
classNode {
intdata;
Node next;
Node(intd)
{
data = d;
next = null;
}
}
voiddeleteAlt()
{
if(head == null)
return;
Node node = head;
while(node != null&& node.next != null) {
/* Change next link of next node */
node.next = node.next.next;
/*Update ref node to new alternate node */
node = node.next;
}
}
/* Utility functions */
/* Inserts a new Node at front of the list. */
publicvoidpush(intnew_data)
{
/* 1 & 2: Allocate the Node &
Put in the data*/
Node new_node = newNode(new_data);
/* 3. Make next of new Node as head */
new_node.next = head;
/* 4. Move the head to point to new Node */
head = new_node;
}
/* Function to print linked list */
voidprintList()
{
Node temp = head;
while(temp != null) {
System.out.print(temp.data + " ");
temp = temp.next;
}
System.out.println();
}
/* Driver program to test above functions */
publicstaticvoidmain(String args[])
{
LinkedList llist = newLinkedList();
/* Constructed Linked List is 1->2->3->4->5->null */
llist.push(5);
llist.push(4);
llist.push(3);
llist.push(2);
llist.push(1);
System.out.println(
"Linked List before calling deleteAlt() ");
llist.printList();
llist.deleteAlt();
System.out.println(
"Linked List after calling deleteAlt() ");
llist.printList();
}
}
/* This code is contributed by Rajat Mishra */
Python3
# Python3 program to remove alternate
# nodes of a linked list
importmath
# A linked list node
classNode:
def__init__(self, data):
self.data =data
self.next=None
# deletes alternate nodes
# of a list starting with head
defdeleteAlt(head):
if(head ==None):
return
# Initialize prev and node to be deleted
prev =head
now =head.next
while(prev !=Noneandnow !=None):
# Change next link of previous node
prev.next=now.next
# Free memory
now =None
# Update prev and node
prev =prev.next
if(prev !=None):
now =prev.next
# UTILITY FUNCTIONS TO TEST fun1() and fun2()
# Given a reference (pointer to pointer) to the head
# of a list and an , push a new node on the front
# of the list.
defpush(head_ref, new_data):
# allocate node
new_node =Node(new_data)
# put in the data
new_node.data =new_data
# link the old list off the new node
new_node.next=head_ref
# move the head to point to the new node
head_ref =new_node
returnhead_ref
# Function to print nodes in a given linked list
defprintList(node):
while(node !=None):
print(node.data, end =" ")
node =node.next
# Driver code
if__name__=='__main__':
# Start with the empty list
head =None
# Using head=push() to construct below list
# 1.2.3.4.5
head =push(head, 5)
head =push(head, 4)
head =push(head, 3)
head =push(head, 2)
head =push(head, 1)
print("List before calling deleteAlt() ")
printList(head)
deleteAlt(head)
print("\nList after calling deleteAlt() ")
printList(head)
# This code is contributed by Srathore
C#
// C# program to delete alternate
// nodes of a linked list
usingSystem;
publicclassLinkedList
{
Node head; // head of list
/* Linked list Node*/
publicclassNode
{
publicintdata;
publicNode next;
publicNode(intd)
{
data = d; next = null;
}
}
voiddeleteAlt()
{
if(head == null)
return;
Node prev = head;
Node now = head.next;
while(prev != null&& now != null)
{
/* Change next link of previous node */
prev.next = now.next;
/* Free node */
now = null;
/*Update prev and now */
prev = prev.next;
if(prev != null)
now = prev.next;
}
}
/* Utility functions */
/* Inserts a new Node at front of the list. */
publicvoidpush(intnew_data)
{
/* 1 & 2: Allocate the Node &
Put in the data*/
Node new_node = newNode(new_data);
/* 3. Make next of new Node as head */
new_node.next = head;
/* 4. Move the head to point to new Node */
head = new_node;
}
/* Function to print linked list */
voidprintList()
{
Node temp = head;
while(temp != null)
{
Console.Write(temp.data+" ");
temp = temp.next;
}
Console.WriteLine();
}
/* Driver code*/
publicstaticvoidMain(String []args)
{
LinkedList llist = newLinkedList();
/* Constructed Linked List is
1->2->3->4->5->null */
llist.push(5);
llist.push(4);
llist.push(3);
llist.push(2);
llist.push(1);
Console.WriteLine("Linked List before"+
"calling deleteAlt() ");
llist.printList();
llist.deleteAlt();
Console.WriteLine("Linked List after"+
"calling deleteAlt() ");
llist.printList();
}
}
// This code has been contributed
// by 29AjayKumar
Javascript
<script>
// Javascript program to delete alternate
// nodes of a linked list
varhead; // head of list
/* Linked list Node */
class Node {
constructor(val) {
this.data = val;
this.next = null;
}
}
functiondeleteAlt() {
if(head == null)
return;
varprev = head;
varnow = head.next;
while(prev != null&& now != null) {
/* Change next link of previous node */
prev.next = now.next;
/* Free node */
now = null;
/* Update prev and now */
prev = prev.next;
if(prev != null)
now = prev.next;
}
}
/* Utility functions */
/* Inserts a new Node at front of the list. */
functionpush(new_data) {
/*
* 1 & 2: Allocate the Node & Put in the data
*/
varnew_node = newNode(new_data);
/* 3. Make next of new Node as head */
new_node.next = head;
/* 4. Move the head to point to new Node */
head = new_node;
}
/* Function to print linked list */
functionprintList() {
vartemp = head;
while(temp != null) {
document.write(temp.data + " ");
temp = temp.next;
}
document.write("<br/>");
}
/* Driver program to test above functions */
/* Constructed Linked List is
1->2->3->4->5->null */
push(5);
push(4);
push(3);
push(2);
push(1);
document.write(
"Linked List before calling deleteAlt() <br/>"
);
printList();
deleteAlt();
document.write(
"Linked List after calling deleteAlt()<br/> "
);
printList();
// This code contributed by gauravrajput1
</script>
Output
List before calling deleteAlt()
1 2 3 4 5
List after calling deleteAlt()
1 3 5
Time Complexity: O(n)
where n is the number of nodes in the given Linked List.
Auxiliary Space: O(1)
As constant extra space is used.
Method 2 (Recursive) Recursive code uses the same approach as method 1. The recursive code is simple and short but causes O(n) recursive function calls for a linked list of size n.
C++
/* deletes alternate nodes of a list starting with head */
voiddeleteAlt(Node *head)
{
if(head == NULL)
return;
Node *node = head->next;
if(node == NULL)
return;
/* Change the next link of head */
head->next = node->next;
/* free memory allocated for node */
free(node);
/* Recursively call for the new next of head */
deleteAlt(head->next);
}
// This code is contributed by rathbhupendra
C
/* deletes alternate nodes of a list starting with head */
voiddeleteAlt(structNode *head)
{
if(head == NULL)
return;
structNode *node = head->next;
if(node == NULL)
return;
/* Change the next link of head */
head->next = node->next;
/* free memory allocated for node */
free(node);
/* Recursively call for the new next of head */
deleteAlt(head->next);
}
Java
/* deletes alternate nodes of a list
starting with head */
staticNode deleteAlt(Node head)
{
if(head == null)
return;
Node node = head.next;
if(node == null)
return;
/* Change the next link of head */
head.next = node.next;
/* Recursively call for the new next of head */
head.next = deleteAlt(head.next);
}
// This code is contributed by Arnab Kundu
Python3
# deletes alternate nodes of a list starting with head
defdeleteAlt(head):
if(head ==None):
return
node =head.next
if(node ==None):
return
# Change the next link of head
head.next=node.next
# free memory allocated for node
#free(node)
# Recursively call for the new next of head
deleteAlt(head.next)
# This code is contributed by Srathore
C#
/* deletes alternate nodes of a list
starting with head */
staticNode deleteAlt(Node head)
{
if(head == null)
return;
Node node = head.next;
if(node == null)
return;
/* Change the next link of head */
head.next = node.next;
/* Recursively call for the new next of head */
head.next = deleteAlt(head.next);
}
// This code is contributed by Arnab Kundu
Javascript
<script>
/* deletes alternate nodes of a list
starting with head */
functiondeleteAlt(head)
{
if(head == null)
return;
varnode = head.next;
if(node == null)
return;
/* Change the next link of head */
head.next = node.next; /* Recursively call for the new next of head */
head.next = deleteAlt(head.next);
}
// This code contributed by aashish1995
</script>
Time Complexity: O(n)
Auxiliary Space: O(1)
As this is a tail recursive function no function call stack is required thus the extra space used is constant.
Please write comments if you find the above code/algorithm incorrect, or find better ways to solve the same problem.
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