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# Swap List Nodes in pairs

Problem Description

NOTE: Your algorithm should use only constant space. You may not modify the values in the list; only nodes themselves can be changed.

Problem Constraints

1 <= |A| <= 106

Input Format

The first and the only argument of input contains a pointer to the head of the given linked list.

Output Format

Example Input

Input 1:

``` A = 1 -> 2 -> 3 -> 4
```

Input 2:

``` A = 7 -> 2 -> 1
```

Example Output

Output 1:

``` 2 -> 1 -> 4 -> 3
```

Output 2:

``` 2 -> 7 -> 1
```

Example Explanation

Explanation 1:

` In the first example (1, 2) and (3, 4) are the adjacent nodes. Swapping them will result in 2 -> 1 -> 4 -> 3`

Explanation 2:

` In the second example, 3rd element i.e. 1 does not have an adjacent node, so it won't be swapped. `

NOTE: You only need to implement the given function. Do not read input, instead use the arguments to the function. Do not print the output, instead return values as specified. Still have a question? Checkout Sample Codes for more details.
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## Discussion

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