Reverse a Linked List

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

Difficulty:

JavaScript Solution

function reverseLinkedList(head) {
  let prev = null, curr = head;
  while(curr){
    let next = curr.next;
    curr.next = prev;
    prev = curr;
    curr = next;
  }
  return prev;
}

Explanation: Iteratively reverse the links by maintaining previous and current pointers.

⏱ O(n) | 💾 O(1)

Java Solution

class Solution {
  public ListNode reverseLinkedList(ListNode head) {
    ListNode prev = null, curr = head;
    while(curr != null){
      ListNode next = curr.next;
      curr.next = prev;
      prev = curr;
      curr = next;
    }
    return prev;
  }
}

Explanation: Iteratively reverse using prev and curr pointers.

⏱ O(n) | 💾 O(1)

Python Solution

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

Explanation: Maintain prev and curr pointers to reverse the linked list iteratively.

⏱ O(n) | 💾 O(1)

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Reverse a Linked List