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 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 interesting question as it has 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 last node.
4) x and/or y may not be present in 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 it to first search x and y in 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. Following are C and Java implementations of this approach.

C++

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/* 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

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C

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/* 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;
    }
}
  
/* Druver 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;
}

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Java

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// 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;
        }
    }
  
    /* Druver 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

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Python

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# 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

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C#

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// 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

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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++

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// 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();
}

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Java

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// 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

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C#

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// 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 */

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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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