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How do you reverse a linked list?

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01

Understand the problem

Explain reversing a linked list.

linked-list
02

Attempt it yourself

Sketch your approach before reading the solution — that's what interviews test.

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03

Study the solution

Iterate with three pointers — prev, curr, next — and at each step point curr.next back to prev, then advance all three. Returns the new head in O(n) time, O(1) space. It can also be done recursively.

Solution ready — 2 min read

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04

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]
05

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