Reverse a stack without using extra space in O(n)

Reverse a Stack without using recursion and extra space. Even the functional Stack is not allowed.

Examples:  

Input : 1->2->3->4
Output : 4->3->2->1

Input : 6->5->4
Output : 4->5->6

We have discussed a way of reversing a stack in the below post.
Reverse a Stack using Recursion

The above solution requires O(n) extra space. We can reverse a stack in O(1) time if we internally represent the stack as a linked list. Reverse a stack would require a reversing of a linked list which can be done with O(n) time and O(1) extra space.
Note that push() and pop() operations still take O(1) time.  

Implementation:

Original Stack
4 3 2 1 

Reversed Stack
1 2 3 4 

Time Complexity: O(n), as we are using a loop to traverse n times. Where n is the number of nodes in the linked list.
Auxiliary Space: O(1), as we are not using any extra space.