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Hooray! It's opposite day. Linked lists go the opposite way today.
**

Write a function for reversing a linked list. Do it in place.

Your function will have one input: the head of the list.

Your function should return the new head of the list.

Here's a sample linked list node class:

class LinkedListNode(object):
def __init__(self, value):
self.value = value
self.next = None

We can do this in space. So don't make a new list; use the existing list nodes!

We can do this is in time.

Careful—even the right *approach* will fail if done in the wrong *order*.

Try drawing a picture of a small linked list and running your function by hand. Does it actually work?

The most obvious edge cases are:

- the list has 0 elements
- the list has 1 element

Does your function correctly handle those cases?

Our first thought might be to build our reversed list "from the beginning," starting with the head of the final *reversed* linked list.

The head of the reversed list will be the *tail* of the input list. To get to that node we'll have to walk through the whole list once ( time). And that's just to get started.

That seems inefficient. **Can we reverse the list while making just one walk from head to tail of the input list?**

We can reverse the list by changing the next pointer of each node. Where should each node's next pointer...point?

Each node's next pointer should point to the *previous* node.

How can we move each node's next pointer to its *previous* node in one pass from head to tail of our current list?

In one pass from head to tail of our input list, we point each node's next pointer to the previous item.

**The order of operations is important here!** We're careful to copy current_node.next into next *before* setting current_node.next to previous_node. Otherwise "stepping forward" at the end could actually mean stepping *back* to previous_node!

def reverse(head_of_list):
current_node = head_of_list
previous_node = None
next_node = None
# Until we have 'fallen off' the end of the list
while current_node:
# Copy a pointer to the next element
# before we overwrite current_node.next
next_node = current_node.next
# Reverse the 'next' pointer
current_node.next = previous_node
# Step forward in the list
previous_node = current_node
current_node = next_node
return previous_node

We return previous_node because when we exit the list, current_node is None. Which means that the last node we visited—previous_node—was the tail of the *original* list, and thus the head of our *reversed* list.

time and space. We pass over the list only once, and maintain a constant number of variables in memory.

This in-place reversal destroys the input linked list. What if we wanted to keep a copy of the original linked list? Write a function for reversing a linked list out-of-place.

It's one of those problems where, even once you know the procedure, it's hard to write a bug-free solution. Drawing it out helps a lot. Write out a sample linked list and walk through your code by hand, step by step, running each operation on your sample input to see if the final output is what you expect. This is a great strategy for *any* coding interview question.

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