How to Reverse A Linked List

Updated August 2026 — full tutorial restored for this URL.

Question: How do you reverse a singly linked list? Example: 1 → 2 → 3 → 4 → null becomes 4 → 3 → 2 → 1 → null.

Companion guide: Everything About Linked List (Python).

1. Node definition
2. Iterative reverse (preferred in interviews)
3. Recursive reverse
4. Python version
5. Complexity

1. Node definition

class ListNode {
  int val;
  ListNode next;
  ListNode(int val) { this.val = val; }
}

2. Iterative reverse (preferred in interviews)

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

Dry-run on 1→2→3: after each step the prev chain grows backward until curr is null; return prev.

3. Recursive reverse

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;
}

4. Python version

def reverse_list(head):
    prev, curr = None, head
    while curr:
        nxt = curr.next
        curr.next = prev
        prev, curr = curr, nxt
    return prev

5. Complexity

• Time O(n), extra space O(1) iterative / O(n) stack recursive
• Watch for cycles if the list might be corrupted