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Reverse Linked List

Easy Linked List Front Middle Back Pointer Original on LeetCode

Reverse Linked List is a easy linked list problem solved with the front middle back pointer pattern. The best approach, optimal (iterative, prev / cur / next), runs in O(n) time and O(1) space. Below are 2 approaches in Java, from recursive up.

Problem

Reverse a singly linked list and return the new head.

Examples

Example 1

Input
head = [1, 2, 3, 4]
Output
[4, 3, 2, 1]

Example 2

Input
head = []
Output
[]

Constraints

  • The list has 0 to 5000 nodes.

Animated walkthrough

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Concepts first, then the problem and every approach, step by step.

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Solutions

Try it yourself first. Then compare: each approach lists its idea, the steps, its time and space, and the Java code.

ApproachTimeSpace
RecursiveO(n)O(n)
Optimal (iterative, prev / cur / next)O(n)O(1)

1Recursive

TimeO(n)
SpaceO(n)Recursion depth n.

Reverse the rest of the list, then make the next node point back at the current one.

  1. If head or head.next is null, return head.
  2. newHead = reverse(head.next); head.next.next = head; head.next = null.
Java
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;
    }
}

2Optimal (iterative, prev / cur / next)

TimeO(n)
SpaceO(1)

Walk the list flipping each link backward. Save next before overwriting cur.next so you can keep going.

  1. prev = null, cur = head.
  2. next = cur.next; cur.next = prev; prev = cur; cur = next.
  3. Return prev.
Java
class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode prev = null, cur = head;
        while (cur != null) {
            ListNode next = cur.next;
            cur.next = prev;
            prev = cur;
            cur = next;
        }
        return prev;
    }
}

Edge cases to test

  • Empty list or one node

Hints

Hint 1

Three pointers: previous, current and the saved next node, so you never lose the rest of the list.

FAQ

What is the best time complexity for Reverse Linked List?

Optimal (iterative, prev / cur / next) runs in O(n) time and O(1) extra space.

Which pattern does Reverse Linked List use?

It is a linked list problem that uses the front middle back pointer pattern. Other problems with the same pattern: Reverse Linked List II, Reverse Nodes in k-Group, Palindrome Linked List.

Is there a brute force solution for Reverse Linked List?

Yes. Recursive takes O(n) time and O(n) space. Reverse the rest of the list, then make the next node point back at the current one.

Which edge cases should I test for Reverse Linked List?

Empty list or one node.