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206. Reverse Linked List 👍

Approach 1: Recursive

  • Time: $O(n)$
  • Space: $O(n)$
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class Solution {
 public:
  ListNode* reverseList(ListNode* head) {
    if (!head || !head->next)
      return head;

    ListNode* newHead = reverseList(head->next);
    head->next->next = head;
    head->next = nullptr;
    return newHead;
  }
};

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class Solution {
  public ListNode reverseList(ListNode head) {
    if (head == null || head.next == null)
      return head;

    ListNode newHead = reverseList(head.next);
    head.next.next = head;
    head.next = null;
    return newHead;
  }
}

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class Solution:
  def reverseList(self, head: ListNode | None) -> ListNode | None:
    if not head or not head.next:
      return head

    newHead = self.reverseList(head.next)
    head.next.next = head
    head.next = None
    return newHead

Approach 2: Iterative

  • Time: $O(n)$
  • Space: $O(1)$
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class Solution {
 public:
  ListNode* reverseList(ListNode* head) {
    ListNode* prev = nullptr;

    while (head != nullptr) {
      ListNode* next = head->next;
      head->next = prev;
      prev = head;
      head = next;
    }

    return prev;
  }
};

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class Solution {
  public ListNode reverseList(ListNode head) {
    ListNode prev = null;

    while (head != null) {
      ListNode next = head.next;
      head.next = prev;
      prev = head;
      head = next;
    }

    return prev;
  }
}

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class Solution:
  def reverseList(self, head: ListNode | None) -> ListNode | None:
    prev = None
    curr = head

    while curr:
      next = curr.next
      curr.next = prev
      prev = curr
      curr = next

    return prev