Reverse Linked List - LeetCode Daily Challenge

Problem Statement

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

Example

Input: head = [3,6,8,2,1,5]
Output: [5,1,2,8,6,3]

Try here before watching the video.

Video Explanation

Java Code

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

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

        head = prev;
        return head;
    }
}

C++ Code

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* prev = nullptr;
        ListNode* curr = head;
        ListNode* next = nullptr;

        while (curr != nullptr) {
            next = curr->next;
            curr->next = prev;
            prev = curr;
            curr = next;
        }

        head = prev;
        return head;
    }
};

Python Code

class Solution:
    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        prev = None
        curr = head
        next = None

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

        head = prev
        return head

Javascript Code

var reverseList = function(head) {
    let prev = null;
    let curr = head;
    let next = null;

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

    head = prev;
    return head;
};

Go Code

func reverseList(head *ListNode) *ListNode {
    var prev *ListNode
    curr := head
    var next *ListNode

    for curr != nil {
        next = curr.Next
        curr.Next = prev
        prev = curr
        curr = next
    }

    head = prev
    return head
}

Complexity Analysis

Time Complexity: O(N), we are iterating over the complete linked list
Space Complexity: O(1), we are not taking any extra space.