Data Structures

Swap nodes by pointers

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

While designing the code, following cases need 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.

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/C++, Java and Python implementations of this approach.

/* 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 begining 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 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();
    }
}

# 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()

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.


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