Reverse a Linked List

Problem

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

Constraints

• 0 ≤ list length ≤ 5000
• −5000 ≤ Node.val ≤ 5000

Approach — Linked List

This is a Linked List problem. The idea: rework the list with careful pointer manipulation — often a dummy head and fast/slow pointers — in a single pass. Work through the reference code below line by line, then re-derive it yourself in the editor — that's how the pattern sticks.

Complexity: O(n) time, O(1) space.

Solution code

Python

class Solution:
    def reverseList(self, head):
        prev = None
        while head:
            nxt = head.next
            head.next = prev
            prev = head
            head = nxt
        return prev

Java

class Solution {
    public 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;
    }
}

Practice it

Reading a solution isn't the same as being able to write it under pressure. Open this problem in the in-browser editor, hide the solution, and solve it from scratch — your code runs against real test cases instantly.