Know this before you start: “Reverse a list iteratively”

Reversing a linked list

Walk with three pointers (prev, current, next): save next, point current back at prev, then step forward. The old tail becomes the head.

The idea

To reverse, every node must point to the one before it. Going node by node, flip current.next to prev.

The danger: once you flip current.next, you've lost the rest of the list. So save it first in NEXT, then flip, then move prev and current one step forward.

See it step by step

Reverse 1 → 2 → 3

prev = None, current = 1: save 2, 1 → None, prev = 1, current = 2
save 3, 2 → 1, prev = 2, current = 3
save None, 3 → 2, prev = 3, current = None → new head 3: 3 → 2 → 1

In code (Python)

class ListNode:
    def __init__(self, val, next=None):
        self.val, self.next = val, next

def reverse(head):
    prev, current = None, head
    while current:
        nxt = current.next      # save the rest
        current.next = prev     # flip the link
        prev, current = current, nxt
    return prev                 # the old tail

def show(head):
    out = []
    while head:
        out.append(head.val)
        head = head.next
    return out

print(show(reverse(ListNode(1, ListNode(2, ListNode(3))))))   # [3, 2, 1]

O(n) time, O(1) space.

When you'll use it

Reversing a whole list or part of it, checking palindrome lists, reorder list, adding numbers stored in reverse.

Common mistakes

Flipping before saving next.
Returning head (now the last node) instead of prev.