Swap nodes in a linked list without swapping data

Difficulty Level : Medium
Last Updated : 19 Mar, 2021

Given a linked list and two keys in it, swap nodes for two given keys. Nodes should be swapped by changing links. Swapping data of nodes may be expensive in many situations when data contains many fields.

It may be assumed that all keys in the linked list are distinct.

Examples:

Input : 10->15->12->13->20->14,  x = 12, y = 20
Output: 10->15->20->13->12->14

Input : 10->15->12->13->20->14,  x = 10, y = 20
Output: 20->15->12->13->10->14

Input : 10->15->12->13->20->14,  x = 12, y = 13
Output: 10->15->13->12->20->14

This may look a simple problem, but is an interesting question as it has the following cases to be handled.

x and y may or may not be adjacent.
Either x or y may be a head node.
Either x or y may be the last node.
x and/or y may not be present in the linked list.

How to write a clean working code that handles all the above possibilities.

The idea is to first search x and y in the given linked list. If any of them is not present, then return. While searching for x and y, keep track of current and previous pointers. First change next of previous pointers, then change next of current pointers.

Below is the implementation of the above approach.

C++

/* This program swaps the nodes of linked list rather
than swapping the field from the nodes.*/
#include <bits/stdc++.h>
using namespace std;
 
/* A linked list node */
class Node {
public:
    int data;
    Node* next;
};
 
/* Function to swap nodes x and y in linked list by
changing links */
void swapNodes(Node** head_ref, int x, int y)
{
    // Nothing to do if x and y are same
    if (x == y)
        return;
 
    // Search for x (keep track of prevX and CurrX
    Node *prevX = NULL, *currX = *head_ref;
    while (currX && currX->data != x) {
        prevX = currX;
        currX = currX->next;
    }
 
    // Search for y (keep track of prevY and CurrY
    Node *prevY = NULL, *currY = *head_ref;
    while (currY && currY->data != y) {
        prevY = currY;
        currY = currY->next;
    }
 
    // If either x or y is not present, nothing to do
    if (currX == NULL || currY == NULL)
        return;
 
    // If x is not head of linked list
    if (prevX != NULL)
        prevX->next = currY;
    else // Else make y as new head
        *head_ref = currY;
 
    // If y is not head of linked list
    if (prevY != NULL)
        prevY->next = currX;
    else // Else make x as new head
        *head_ref = currX;
 
    // Swap next pointers
    Node* temp = currY->next;
    currY->next = currX->next;
    currX->next = temp;
}
 
/* Function to add a node at the beginning of List */
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 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 */
void printList(Node* node)
{
    while (node != NULL) {
        cout << node->data << " ";
        node = node->next;
    }
}
 
/* Driver program to test above function */
int main()
{
    Node* start = NULL;
 
    /* The constructed linked list is:
    1->2->3->4->5->6->7 */
    push(&start, 7);
    push(&start, 6);
    push(&start, 5);
    push(&start, 4);
    push(&start, 3);
    push(&start, 2);
    push(&start, 1);
 
    cout << "Linked list before calling swapNodes() ";
    printList(start);
 
    swapNodes(&start, 4, 3);
 
    cout << "\nLinked list after calling swapNodes() ";
    printList(start);
 
    return 0;
}
 
// This is code is contributed by rathbhupendra

C

/* This program swaps the nodes of linked list rather
   than swapping the field from the nodes.*/
 
#include <stdio.h>
#include <stdlib.h>
 
/* A linked list node */
struct Node {
    int data;
    struct Node* next;
};
 
/* Function to swap nodes x and y in linked list by
   changing links */
void swapNodes(struct Node** head_ref, int x, int y)
{
    // Nothing to do if x and y are same
    if (x == y)
        return;
 
    // Search for x (keep track of prevX and CurrX
    struct Node *prevX = NULL, *currX = *head_ref;
    while (currX && currX->data != x) {
        prevX = currX;
        currX = currX->next;
    }
 
    // Search for y (keep track of prevY and CurrY
    struct Node *prevY = NULL, *currY = *head_ref;
    while (currY && currY->data != y) {
        prevY = currY;
        currY = currY->next;
    }
 
    // If either x or y is not present, nothing to do
    if (currX == NULL || currY == NULL)
        return;
 
    // If x is not head of linked list
    if (prevX != NULL)
        prevX->next = currY;
    else // Else make y as new head
        *head_ref = currY;
 
    // If y is not head of linked list
    if (prevY != NULL)
        prevY->next = currX;
    else // Else make x as new head
        *head_ref = currX;
 
    // Swap next pointers
    struct Node* temp = currY->next;
    currY->next = currX->next;
    currX->next = temp;
}
 
/* Function to add a node at the beginning of List */
void push(struct Node** head_ref, int new_data)
{
    /* allocate node */
    struct Node* new_node
        = (struct Node*)malloc(sizeof(struct Node));
 
    /* 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 */
void printList(struct Node* node)
{
    while (node != NULL) {
        printf("%d ", node->data);
        node = node->next;
    }
}
 
/* Driver program to test above function */
int main()
{
    struct Node* start = NULL;
 
    /* The constructed linked list is:
     1->2->3->4->5->6->7 */
    push(&start, 7);
    push(&start, 6);
    push(&start, 5);
    push(&start, 4);
    push(&start, 3);
    push(&start, 2);
    push(&start, 1);
 
    printf("\n Linked list before calling swapNodes() ");
    printList(start);
 
    swapNodes(&start, 4, 3);
 
    printf("\n Linked list after calling swapNodes() ");
    printList(start);
 
    return 0;
}

Java

// Java program to swap two given nodes of a linked list
 
class Node {
    int data;
    Node next;
    Node(int d)
    {
        data = d;
        next = null;
    }
}
 
class LinkedList {
    Node head; // head of list
 
    /* Function to swap Nodes x and y in linked list by
       changing links */
    public void swapNodes(int x, int y)
    {
        // Nothing to do if x and y are same
        if (x == y)
            return;
 
        // Search for x (keep track of prevX and CurrX)
        Node prevX = null, currX = head;
        while (currX != null && currX.data != x) {
            prevX = currX;
            currX = currX.next;
        }
 
        // Search for y (keep track of prevY and currY)
        Node prevY = null, currY = head;
        while (currY != null && currY.data != y) {
            prevY = currY;
            currY = currY.next;
        }
 
        // If either x or y is not present, nothing to do
        if (currX == null || currY == null)
            return;
 
        // If x is not head of linked list
        if (prevX != null)
            prevX.next = currY;
        else // make y the new head
            head = currY;
 
        // If y is not head of linked list
        if (prevY != null)
            prevY.next = currX;
        else // make x the new head
            head = currX;
 
        // Swap next pointers
        Node temp = currX.next;
        currX.next = currY.next;
        currY.next = temp;
    }
 
    /* Function to add Node at beginning of list. */
    public void push(int new_data)
    {
        /* 1. alloc the Node and put the data */
        Node new_Node = new Node(new_data);
 
        /* 2. Make next of new Node as head */
        new_Node.next = head;
 
        /* 3. Move the head to point to new Node */
        head = new_Node;
    }
 
    /* This function prints contents of linked list starting
       from the given Node */
    public void printList()
    {
        Node tNode = head;
        while (tNode != null) {
            System.out.print(tNode.data + " ");
            tNode = tNode.next;
        }
    }
 
    /* Driver program to test above function */
    public static void main(String[] args)
    {
        LinkedList llist = new LinkedList();
 
        /* The constructed linked list is:
            1->2->3->4->5->6->7 */
        llist.push(7);
        llist.push(6);
        llist.push(5);
        llist.push(4);
        llist.push(3);
        llist.push(2);
        llist.push(1);
 
        System.out.print(
            "\n Linked list before calling swapNodes() ");
        llist.printList();
 
        llist.swapNodes(4, 3);
 
        System.out.print(
            "\n Linked list after calling swapNodes() ");
        llist.printList();
    }
}
// This code is contributed by Rajat Mishra

Python

# Python program to swap two given nodes of a linked list
class LinkedList(object):
    def __init__(self):
        self.head = None
 
    # head of list
    class Node(object):
        def __init__(self, d):
            self.data = d
            self.next = None
 
    # Function to swap Nodes x and y in linked list by
    # changing links
    def swapNodes(self, x, y):
 
        # Nothing to do if x and y are same
        if x == y:
            return
 
        # Search for x (keep track of prevX and CurrX)
        prevX = None
        currX = self.head
        while currX != None and currX.data != x:
            prevX = currX
            currX = currX.next
 
        # Search for y (keep track of prevY and currY)
        prevY = None
        currY = self.head
        while currY != None and currY.data != y:
            prevY = currY
            currY = currY.next
 
        # If either x or y is not present, nothing to do
        if currX == None or currY == None:
            return
        # If x is not head of linked list
        if prevX != None:
            prevX.next = currY
        else:  # make y the new head
            self.head = currY
 
        # If y is not head of linked list
        if prevY != None:
            prevY.next = currX
        else:  # make x the new head
            self.head = currX
 
        # Swap next pointers
        temp = currX.next
        currX.next = currY.next
        currY.next = temp
 
    # Function to add Node at beginning of list.
    def push(self, new_data):
 
        # 1. alloc the Node and put the data
        new_Node = self.Node(new_data)
 
        # 2. Make next of new Node as head
        new_Node.next = self.head
 
        # 3. Move the head to point to new Node
        self.head = new_Node
 
    # This function prints contents of linked list starting
    # from the given Node
    def printList(self):
        tNode = self.head
        while tNode != None:
            print tNode.data,
            tNode = tNode.next
 
 
# Driver program to test above function
llist = LinkedList()
 
# The constructed linked list is:
# 1->2->3->4->5->6->7
llist.push(7)
llist.push(6)
llist.push(5)
llist.push(4)
llist.push(3)
llist.push(2)
llist.push(1)
print "Linked list before calling swapNodes() "
llist.printList()
llist.swapNodes(4, 3)
print "\nLinked list after calling swapNodes() "
llist.printList()
 
# This code is contributed by BHAVYA JAIN

C#

// C# program to swap two given
// nodes of a linked list
using System;
 
class Node {
    public int data;
    public Node next;
    public Node(int d)
    {
        data = d;
        next = null;
    }
}
 
public class LinkedList {
    Node head; // head of list
 
    /* Function to swap Nodes x and y in
    linked list by changing links */
    public void swapNodes(int x, int y)
    {
        // Nothing to do if x and y are same
        if (x == y)
            return;
 
        // Search for x (keep track of prevX and CurrX)
        Node prevX = null, currX = head;
        while (currX != null && currX.data != x) {
            prevX = currX;
            currX = currX.next;
        }
 
        // Search for y (keep track of prevY and currY)
        Node prevY = null, currY = head;
        while (currY != null && currY.data != y) {
            prevY = currY;
            currY = currY.next;
        }
 
        // If either x or y is not present, nothing to do
        if (currX == null || currY == null)
            return;
 
        // If x is not head of linked list
        if (prevX != null)
            prevX.next = currY;
        else // make y the new head
            head = currY;
 
        // If y is not head of linked list
        if (prevY != null)
            prevY.next = currX;
        else // make x the new head
            head = currX;
 
        // Swap next pointers
        Node temp = currX.next;
        currX.next = currY.next;
        currY.next = temp;
    }
 
    /* Function to add Node at beginning of list. */
    public void push(int new_data)
    {
        /* 1. alloc the Node and put the data */
        Node new_Node = new Node(new_data);
 
        /* 2. Make next of new Node as head */
        new_Node.next = head;
 
        /* 3. Move the head to point to new Node */
        head = new_Node;
    }
 
    /* This function prints contents of linked list
    starting from the given Node */
    public void printList()
    {
        Node tNode = head;
        while (tNode != null) {
            Console.Write(tNode.data + " ");
            tNode = tNode.next;
        }
    }
 
    /* Driver code */
    public static void Main(String[] args)
    {
        LinkedList llist = new LinkedList();
 
        /* The constructed linked list is:
            1->2->3->4->5->6->7 */
        llist.push(7);
        llist.push(6);
        llist.push(5);
        llist.push(4);
        llist.push(3);
        llist.push(2);
        llist.push(1);
 
        Console.Write(
            "\n Linked list before calling swapNodes() ");
        llist.printList();
 
        llist.swapNodes(4, 3);
 
        Console.Write(
            "\n Linked list after calling swapNodes() ");
        llist.printList();
    }
}
 
// This code is contributed by 29AjayKumar

Output

Linked list before calling swapNodes() 1 2 3 4 5 6 7 
Linked list after calling swapNodes() 1 2 4 3 5 6 7

Optimizations: The above code can be optimized to search x and y in single traversal. Two loops are used to keep program simple.

Simpler approach –

C++

// C++ program to swap two given nodes of a linked list
#include <iostream>
 
using namespace std;
 
// A linked list node class
class Node {
 
public:
    int data;
    class Node* next;
 
    // constructor
    Node(int val, Node* next)
        : data(val)
        , next(next)
    {
    }
 
    // print list from this
    // to last till null
    void printList()
    {
 
        Node* node = this;
 
        while (node != NULL) {
 
            cout << node->data << " ";
            node = node->next;
        }
 
        cout << endl;
    }
};
 
// Function to add a node
// at the beginning of List
void push(Node** head_ref, int new_data)
{
 
    // allocate node
    (*head_ref) = new Node(new_data, *head_ref);
}
 
void swap(Node*& a, Node*& b)
{
 
    Node* temp = a;
    a = b;
    b = temp;
}
 
void swapNodes(Node** head_ref, int x, int y)
{
 
    // Nothing to do if x and y are same
    if (x == y)
        return;
 
    Node **a = NULL, **b = NULL;
 
    // search for x and y in the linked list
    // and store therir pointer in a and b
    while (*head_ref) {
 
        if ((*head_ref)->data == x) {
            a = head_ref;
        }
 
        else if ((*head_ref)->data == y) {
            b = head_ref;
        }
 
        head_ref = &((*head_ref)->next);
    }
 
    // if we have found both a and b
    // in the linked list swap current
    // pointer and next pointer of these
    if (a && b) {
 
        swap(*a, *b);
        swap(((*a)->next), ((*b)->next));
    }
}
 
// Driver code
int main()
{
 
    Node* start = NULL;
 
    // The constructed linked list is:
    // 1->2->3->4->5->6->7
    push(&start, 7);
    push(&start, 6);
    push(&start, 5);
    push(&start, 4);
    push(&start, 3);
    push(&start, 2);
    push(&start, 1);
 
    cout << "Linked list before calling swapNodes() ";
    start->printList();
 
    swapNodes(&start, 6, 1);
 
    cout << "Linked list after calling swapNodes() ";
    start->printList();
}

Java

// Java program to swap two given nodes of a linked list
public class Solution {
 
    // Represent a node of the singly linked list
    class Node {
        int data;
        Node next;
 
        public Node(int data)
        {
            this.data = data;
            this.next = null;
        }
    }
 
    // Represent the head and tail of the singly linked list
    public Node head = null;
    public Node tail = null;
 
    // addNode() will add a new node to the list
    public void addNode(int data)
    {
        // Create a new node
        Node newNode = new Node(data);
 
        // Checks if the list is empty
        if (head == null) {
            // If list is empty, both head and
            // tail will point to new node
            head = newNode;
            tail = newNode;
        }
        else {
            // newNode will be added after tail such that
            // tail's next will point to newNode
            tail.next = newNode;
            // newNode will become new tail of the list
            tail = newNode;
        }
    }
 
    // swap() will swap the given two nodes
    public void swap(int n1, int n2)
    {
        Node prevNode1 = null, prevNode2 = null,
             node1 = head, node2 = head;
 
        // Checks if list is empty
        if (head == null) {
            return;
        }
 
        // If n1 and n2 are equal, then
        // list will remain the same
        if (n1 == n2)
            return;
 
        // Search for node1
        while (node1 != null && node1.data != n1) {
            prevNode1 = node1;
            node1 = node1.next;
        }
 
        // Search for node2
        while (node2 != null && node2.data != n2) {
            prevNode2 = node2;
            node2 = node2.next;
        }
 
        if (node1 != null && node2 != null) {
 
            // If previous node to node1 is not null then,
            // it will point to node2
            if (prevNode1 != null)
                prevNode1.next = node2;
            else
                head = node2;
 
            // If previous node to node2 is not null then,
            // it will point to node1
            if (prevNode2 != null)
                prevNode2.next = node1;
            else
                head = node1;
 
            // Swaps the next nodes of node1 and node2
            Node temp = node1.next;
            node1.next = node2.next;
            node2.next = temp;
        }
        else {
            System.out.println("Swapping is not possible");
        }
    }
 
    // display() will display all the
    // nodes present in the list
    public void display()
    {
        // Node current will point to head
        Node current = head;
 
        if (head == null) {
            System.out.println("List is empty");
            return;
        }
        while (current != null) {
            // Prints each node by incrementing pointer
            System.out.print(current.data + " ");
            current = current.next;
        }
        System.out.println();
    }
 
    public static void main(String[] args)
    {
 
        Solution sList = new Solution();
 
        // Add nodes to the list
        sList.addNode(1);
        sList.addNode(2);
        sList.addNode(3);
        sList.addNode(4);
        sList.addNode(5);
        sList.addNode(6);
        sList.addNode(7);
 
        System.out.println("Original list: ");
        sList.display();
 
        // Swaps the node 2 with node 5
        sList.swap(6, 1);
 
        System.out.println("List after swapping nodes: ");
        sList.display();
    }
}

Python3

# Python3 program to swap two given
# nodes of a linked list
 
# A linked list node class
 
 
class Node:
 
    # constructor
    def __init__(self, val=None, next1=None):
        self.data = val
        self.next = next1
 
    # print list from this
    # to last till None
    def printList(self):
 
        node = self
 
        while (node != None):
            print(node.data, end=" ")
            node = node.next
        print(" ")
 
# Function to add a node
# at the beginning of List
 
 
def push(head_ref, new_data):
 
    # allocate node
    (head_ref) = Node(new_data, head_ref)
    return head_ref
 
 
def swapNodes(head_ref, x, y):
    head = head_ref
 
    # Nothing to do if x and y are same
    if (x == y):
        return None
 
    a = None
    b = None
 
    # search for x and y in the linked list
    # and store therir pointer in a and b
    while (head_ref.next != None):
 
        if ((head_ref.next).data == x):
            a = head_ref
 
        elif ((head_ref.next).data == y):
            b = head_ref
 
        head_ref = ((head_ref).next)
 
    # if we have found both a and b
    # in the linked list swap current
    # pointer and next pointer of these
    if (a != None and b != None):
        temp = a.next
        a.next = b.next
        b.next = temp
        temp = a.next.next
        a.next.next = b.next.next
        b.next.next = temp
 
    return head
 
# Driver code
 
 
start = None
 
# The constructed linked list is:
# 1.2.3.4.5.6.7
start = push(start, 7)
start = push(start, 6)
start = push(start, 5)
start = push(start, 4)
start = push(start, 3)
start = push(start, 2)
start = push(start, 1)
 
print("Linked list before calling swapNodes() ")
start.printList()
 
start = swapNodes(start, 6, 1)
 
print("Linked list after calling swapNodes() ")
start.printList()
 
# This code is contributed by Arnab Kundu

C#

// C# program to swap two
// given nodes of a linked list
using System;
 
class GFG {
    // A linked list node class
    public class Node {
 
        public int data;
        public Node next;
 
        // constructor
        public Node(int val, Node next1)
        {
            data = val;
            next = next1;
        }
 
        // print list from this
        // to last till null
        public void printList()
        {
 
            Node node = this;
 
            while (node != null) {
                Console.Write(node.data + " ");
                node = node.next;
            }
            Console.WriteLine();
        }
    }
 
    // Function to add a node
    // at the beginning of List
    static Node push(Node head_ref, int new_data)
    {
 
        // allocate node
        (head_ref) = new Node(new_data, head_ref);
        return head_ref;
    }
 
    static Node swapNodes(Node head_ref, int x, int y)
    {
        Node head = head_ref;
 
        // Nothing to do if x and y are same
        if (x == y)
            return null;
 
        Node a = null, b = null;
 
        // search for x and y in the linked list
        // and store therir pointer in a and b
        while (head_ref.next != null) {
 
            if ((head_ref.next).data == x) {
                a = head_ref;
            }
 
            else if ((head_ref.next).data == y) {
                b = head_ref;
            }
 
            head_ref = ((head_ref).next);
        }
 
        // if we have found both a and b
        // in the linked list swap current
        // pointer and next pointer of these
        if (a != null && b != null) {
            Node temp = a.next;
            a.next = b.next;
            b.next = temp;
            temp = a.next.next;
            a.next.next = b.next.next;
            b.next.next = temp;
        }
        return head;
    }
 
    // Driver code
    public static void Main()
    {
 
        Node start = null;
 
        // The constructed linked list is:
        // 1.2.3.4.5.6.7
        start = push(start, 7);
        start = push(start, 6);
        start = push(start, 5);
        start = push(start, 4);
        start = push(start, 3);
        start = push(start, 2);
        start = push(start, 1);
 
        Console.Write(
            "Linked list before calling swapNodes() ");
        start.printList();
 
        start = swapNodes(start, 6, 1);
 
        Console.Write(
            "Linked list after calling swapNodes() ");
        start.printList();
    }
}
 
/* This code contributed by PrinciRaj1992 */

Output

Linked list before calling swapNodes() 1 2 3 4 5 6 7 
Linked list after calling swapNodes() 6 2 3 4 5 1 7

https://www.youtube.com/watch?v=V4ZHvhvVmSE

This article is contributed by Gautam. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

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