Given a singly linked list, write a function to swap elements pairwise.
Input: 1->2->3->4->5->6->NULL
Output: 2->1->4->3->6->5->NULL
Input: 1->2->3->4->5->NULL
Output: 2->1->4->3->5->NULL
Input: 1->NULL
Output: 1->NULL
For example, if the linked list is 1->2->3->4->5 then the function should change it to 2->1->4->3->5, and if the linked list is then the function should change it to.
METHOD 1 (Iterative):
Start from the head node and traverse the list. While traversing swap data of each node with its next node's data.
Below is the implementation of the above approach:
// JavaScript program to pairwise swap
// elements of a linked list
// head of list
let head;
// Linked list Node
class Node {
constructor(val) {
this.data = val;
this.next = null;
}
}
function pairWiseSwap() {
let temp = head;
/* Traverse only till there are
atleast 2 nodes left */
while (temp != null &&
temp.next != null) {
// Swap the data
let k = temp.data;
temp.data = temp.next.data;
temp.next.data = k;
temp = temp.next.next;
}
}
// Utility functions
// Inserts a new Node at front
// of the list.
function push(new_data) {
/* 1 & 2: Allocate the Node &
Put in the data */
let new_node = new Node(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
function printList() {
let temp = head;
while (temp != null) {
console.log(temp.data);
temp = temp.next;
}
}
// Driver code
// Created Linked List
// 1 -> 2 -> 3 -> 4 -> 5
push(5);
push(4);
push(3);
push(2);
push(1);
console.log("Linked List before calling pairWiseSwap()");
printList();
pairWiseSwap();
console.log("Linked List after calling pairWiseSwap()");
printList();
// This code is contributed by todaysgaurav
Output
Linked List before calling pairWiseSwap() 1 2 3 4 5 Linked List after calling pairWiseSwap() 2 1 4 3 5
Complexity Analysis:
- Time complexity: O(n), as we are traversing over the linked list of size N using a while loop.
- Auxiliary Space: O(1), as we are not using any extra space.
METHOD 2 (Recursive):
If there are 2 or more than 2 nodes in Linked List then swap the first two nodes and recursively call for rest of the list.
Below image is a dry run of the above approach:
Below is the implementation of the above approach:
// JavaScript program to pairwise swap
// elements of a linked list
// head of list
let head;
// Linked list Node
class Node {
constructor(val) {
this.data = val;
this.next = null;
}
}
/* Recursive function to pairwise swap
elements of a linked list */
function pairWiseSwap(head) {
/* There must be at-least two nodes
in the list */
if (head != null &&
head.next != null) {
/* Swap the node's data with data
of next node */
swap(head.data, head.next.data);
/* Call pairWiseSwap() for rest of
the list */
pairWiseSwap(head.next.next);
}
}
// Utility functions
// Inserts a new Node at front
// of the list.
function push(new_data) {
/* 1 & 2: Allocate the Node &
Put in the data */
let new_node = new Node(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
function printList() {
let temp = head;
while (temp != null) {
console.log(temp.data);
temp = temp.next;
}
}
// Driver code
// Created Linked List
// 1 -> 2 -> 3 -> 4 -> 5
push(5);
push(4);
push(3);
push(2);
push(1);
console.log("Linked List before calling pairWiseSwap()");
printList();
pairWiseSwap();
console.log("Linked List after calling pairWiseSwap()");
printList();
// This code is contributed by todaysgaurav
Output
Linked List before calling pairWiseSwap() 1 2 3 4 5 Linked List after calling pairWiseSwap() 1 2 3 4 5
Complexity Analysis:
- Time complexity: O(n), as we are traversing over the linked list of size N using recursion.
- Auxiliary Space: O(1), as we are not using any extra space.
The solution provided there swaps data of nodes. If data contains many fields, there will be many swap operations. See this for an implementation that changes links rather than swapping data.
Please refer complete article on Pairwise swap elements of a given linked list for more details!