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You are given a singly Linked List of integers. Your task is to return true if the given singly linked list is a palindrome otherwise returns false.

```
The given linked list is 1 -> 2 -> 3 -> 2-> 1-> NULL.
It is a palindrome linked list because the given linked list has the same order of elements when traversed forwards and backward.
```

```
Can you solve the problem in O(N) time complexity and O(1) space complexity iteratively?
```

Detailed explanation

```
1 <= T <= 10
0 <= L <= 10^5
1 <= data <= 10^9 and data != -1
Where L is the number of nodes in the Linked List.
Time Limit: 1 sec
```

```
2
1 2 3 4 5 6 -1
1 2 1 -1
```

```
false
true
```

```
For the first test case, it is not a palindrome because Linked List doesn't have the same order of elements when traversed forwards and backwards.
For the second test case, it is a palindrome linked list because a Linked List has the same order of elements when traversed forwards and backwards.
```

```
2
1 -1
1 10 45 10 1 -1
```

```
true
true
```