## Problem

Given a linked list, swap every two adjacent nodes and return its head.

You may not modify the values in the list’s nodes, only nodes itself may be changed.

Example:

```
Given 1->2->3->4, you should return the list as 2->1->4->3.
```

## Thought Process

- For every node pair: front and back
- front’s next will be pointed to the back’s next
- back’s next will be pointed to the front
- front’s previous node’s next will be pointed to back
- Setting up a dummy head node will be helpful

## White Board

Below is the white board:

## Code

```
class LN:
def __init__(self, val):
self.val = val
self.next = None
class Sol:
def swapPairs(self, head):
# create a dummy head, use -1 as default
dh = LN(-1)
dh.next = head
# create a pointer index based off dh
idx = dh
while (idx.next and idx.next.next):
# initialize the nodes for pair swap
n1 = idx.next
n2 = n1.next
nxt = n2.next
# swap pairs
n2.next = n1
n1.next = nxt
idx.next = n2
# updated index
dh = idx.next.next
# return the head node
return dh.next
```

```
```

## Complexity

- Time complexity is O(n), space complexity is O(1)