English
206. Reverse Linked List 
Problem Statement:
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?
Solution:
java
public ListNode reverseList(ListNode head) {
if (head == null || head.next == null)
return head;
ListNode newHead = reverseList(head.next);
head.next.next = head;
head.next = null;
return newHead;
}
public ListNode reverseIterate(ListNode head) {
if (head == null || head.next == null)
return head;
ListNode prevNode = head,
currentNode = head.next;
while (currentNode != null) {
ListNode nextNode = currentNode.next;
currentNode.next = prevNode;
prevNode = currentNode;
currentNode = nextNode;
}
head.next = null;
head = prevNode;
return prevNode;
}
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