How do you reverse a linked list?

Q: How do you reverse a linked list?

**Intuition.** Walk the list once, flipping each next pointer to point **backwards**. You carry three references — prev , curr , next — so you never lose the rest of the list when you rewire a node. flip each arrow: curr.next = prev, then step forward 1 2 <circle

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Understand the problem

Explain reversing a linked list.

linked-list

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Read the code

Iterative reversal

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

def reverse_list(head):
    prev, curr = None, head
    while curr:
        nxt = curr.next     # save next
        curr.next = prev    # flip
        prev = curr         # advance
        curr = nxt
    return prev             # new head


# --- demo ---
def build(vals):
    head = None
    for v in reversed(vals): head = ListNode(v, head)
    return head
def to_list(head):
    out = []
    while head: out.append(head.val); head = head.next
    return out

print(to_list(reverse_list(build([1, 2, 3]))))   # [3, 2, 1]

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