# Swap nodes in a linked list without swapping data

Python Algorithm - Swap nodes in a linked list without swapping data - Linked List - Given a linked list and two keys in it, swap nodes for two given keys

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.

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 python implementation of this approach.

### Python Programming:

``````# Python program to swap two given nodes of a linked list
def __init__(self):

class Node(object):
def __init__(self, d):
self.data = d
self.next = None

# Function to swap Nodes x and y in linked list by
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
while currX != None and currX.data != x:
prevX = currX
currX = currX.next

# Search for y (keep track of prevY and currY)
prevY = None
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 prevX != None:
prevX.next = currY
else: #make y the new head

if prevY != None:
prevY.next = currX
else: # make x the new head

# 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

# 3. Move the head to point to new Node

# This function prints contents of linked list starting
# from the given Node
def printList(self):
while tNode != None:
print tNode.data,
tNode = tNode.next

# Driver program to test above function

# 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``````

### Output:

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