206. Reverse Linked List

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

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Example 1:

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

ex1

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Example 2:

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

ex2

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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?

Solution

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/**
 * 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; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        
        ListNode prev = null;

        while (head != null) {
            ListNode nextNode = head.next;
            
            head.next = prev;
            
            prev = head;
            
            head = nextNode;
        }
        
        return prev;
    }
}

Solution#

Solution