Reverse Linked List

Easy

Given the head of a singly linked list, reverse the list and return the reversed list.

Examples

InputOutput
head = [1,2,3,4,5][5,4,3,2,1]
head = [1,2][2,1]

Python Solution

def reverse_list(head: ListNode | None) -> ListNode | None:
    prev = None
    while head:
        next_node = head.next
        head.next = prev
        prev = head
        head = next_node
    return prev

JavaScript Solution

function reverseList(head) {
  let prev = null;
  while (head) {
    const next = head.next;
    head.next = prev;
    prev = head;
    head = next;
  }
  return prev;
}

Step-by-Step Explanation

  1. Use three pointers: prev (initially null), current (head), and next.
  2. For each node: save next, point current.next to prev, advance prev and current.
  3. Continue until head is null. prev will be the new head.

Complexity Analysis

TimeO(n)
SpaceO(1)

Tags

Linked ListRecursion

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