Reverse Linked List

LeetCode 206 | Difficulty: Easy

Easy

Problem Description

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

Example 1:

Input: head = [1,2,3,4,5]
Output: [5,4,3,2,1]

Example 2:

Input: head = [1,2]
Output: [2,1]

Example 3:

Input: head = []
Output: []

Constraints:

- The number of nodes in the list is the range `[0, 5000]`.

- `-5000 <= Node.val <= 5000`

Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both?

Topics: Linked List, Recursion

---

Approach

Linked List

Use pointer manipulation. Common techniques: dummy head node to simplify edge cases, fast/slow pointers for cycle detection and middle finding, prev/curr/next pattern for reversal.

When to use

In-place list manipulation, cycle detection, merging lists, finding the k-th node.

---

Solutions

Solution 1: C# (Best: 96 ms)

Metric	Value
Runtime	96 ms
Memory	23.8 MB
Date	2019-12-15

Solution

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode ReverseList(ListNode head) {
            if (head == null || head.next == null)
    {
        return head;
    }
    ListNode rev = null;
    while (head!=null)
    {
        ListNode temp = head.next;
        head.next = rev;
        rev = head;
        head = temp;
    }
    return rev;
    }
}

📜 1 more C# submission(s)

Submission (2017-09-22) — 172 ms, N/A

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode ReverseList(ListNode head) {
        ListNode rev = null;
        while (head != null)
        {
            ListNode temp = head;
            head = head.next;
            temp.next = rev;
            rev = temp;
        }
        return rev;
    }
}

---

Complexity Analysis

Approach	Time	Space
Linked List	$O(n)$	$O(1)$

---

Interview Tips

Key Points

• Start by clarifying edge cases: empty input, single element, all duplicates.
• Draw the pointer changes before coding. A dummy head node simplifies edge cases.