Swap nodes in a linked list without swapping data

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

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

We strongly recommend to minimize your browser and try this yourself first.
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, 3); 
  
    cout << "Linked list after calling swapNodes() "; 
    start->printList(); 
} 

Java

// Java program to swap two given nodes of a linked list 
class Soluiton 
{ 
// A linked list node class  
static class Node {  
  
    int data;  
    Node next;  
  
    // constructor  
    Node(int val, Node next1)  
    {   
        data=val; next=next1; 
    }  
  
    // print list from this  
    // to last till null  
    void printList()  
    {  
  
        Node node = this;  
  
        while (node != null) {  
  
            System.out.print(node.data+" ");  
            node = node.next;  
        }  
        System.out.println();  
    }  
} 
  
// 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(String args[]) 
{  
  
    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);  
  
    System.out.print("Linked list before calling swapNodes() ");  
    start.printList();  
  
    start=swapNodes(start, 6, 3);  
  
    System.out.print("Linked list after calling swapNodes() ");  
    start.printList();  
}  
} 
//contributed by Arnab Kundu 

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, 3)  
  
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, 3);  
  
        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() 1 2 6 4 5 3 7

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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