Reverse Linked List

Reverse a singly linked list.

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Hint:

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

Iteratively:

/**
 * 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) {
        ListNode first = null;
        ListNode second = head;
        while(second != null){
            ListNode tmp = second.next;
            second.next = first;
            first = second;
            second = tmp;
        }
        return first;
    }
}

recursively:

/**
 * 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 nextNode = head.next;
        head.next = null;
        ListNode newHead = reverseList(nextNode);
        nextNode.next = head;
        return newHead;
    }
}