206. Reverse Linked List

Description

Reverse a singly linked list.

Constraints

Approach

Links

• GeeksforGeeks

• Leetcode

• ProgramCreek

• YouTube

Examples

Example 1

Input: 1->2->3->4->5->NULL

Output: 5->4->3->2->1->NULL

Solutions

ListNode

// Definition for singly-linked list.
public class ListNode {
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) {
        this.val = val;
    }
    ListNode(int val, ListNode next) {
        this.val = val;
        this.next = next;
    }
}

Solution 1

/**
 * Time complexity : 
 * Space complexity : 
 */

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

Solution 2

/**
 * Time complexity : 
 * Space complexity : 
 */

class Solution {
    public ListNode reverseList(ListNode head) {
        if(head == null || head.next == null) {
            return head;
        }
        
        ListNode p = reverseList(head.next);
        head.next.next = head;
        head.next = null;
        
        return p;
    }
}

Follow up

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