206. Reverse Linked List

Reverse a singly linked list.

Hint:

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

Solution:

Method 1: recursively

The idea is to reverse the list from the second node, and attaches the head to the end.

For example:
1 -> 2 -> 3 -> 4
=> 1, 2 -> 3-> 4
=> 2 -> 3 -> 4 -> 1



Code:

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

Method 2: iteratively

When we check a node, we want to set its next pointer to its previous node.

Therefore we use a prev node to record its previous node when we go through the list.


Code:

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