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Given a singly linked list, modify the value of **first half nodes** such that :

- 1st node’s new value = the last node’s value - first node’s current value
- 2nd node’s new value = the second last node’s value - 2nd node’s current value,

and so on …

NOTE :

* If the length L of linked list is odd, then the first half implies at first floor(L/2) nodes. So, if L = 5, the first half refers to first 2 nodes.

* If the length L of linked list is even, then the first half implies at first L/2 nodes. So, if L = 4, the first half refers to first 2 nodes.

**Example :**

Given linked list `1 -> 2 -> 3 -> 4 -> 5`

,

You should return `4 -> 2 -> 3 -> 4 -> 5`

as

```
for first node, 5 - 1 = 4
for second node, 4 - 2 = 2
```

Try to solve the problem using constant extra space.

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