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/**

* Reverse a singly linked list.

*

* Hint:

* A linked list can be reversed either iteratively or recursively.

* Could you implement both?

*

*/

/**

* Definition for singly-linked list.

* public class ListNode {

* int val;

* ListNode next;

* ListNode(int x) { val = x; }

* }

*/

public class ReverseLinkedList206 {

public ListNode reverseList(ListNode head) {

if (head == null || head.next == null) return head;

ListNode dummy = new ListNode(0);

ListNode tail = null;

while (head != null) {

ListNode t = head;

head = head.next;

tail = dummy.next;

dummy.next = t;

dummy.next.next = tail;

}

return dummy.next;

}

public ListNode reverseList2(ListNode head) {

if (head == null || head.next == null) return head;

ListNode dummy = new ListNode(0);

helper(head, dummy);

return dummy.next;

}

private void helper(ListNode head, ListNode dummy) {

if (head == null) return;

ListNode curr = head;

head = head.next;

ListNode tail = dummy.next;

dummy.next = curr;

dummy.next.next = tail;

helper(head, dummy);

}

public ListNode reverseList3(ListNode head) {

return helper2(head, null);

}

private ListNode helper2(ListNode head, ListNode h) {

if (head == null) return h;

ListNode tail = head.next;

head.next = h;

return helper2(tail, head);

}

/**

* https://leetcode.com/problems/reverse-linked-list/solution/

*/

public ListNode reverseList4(ListNode head) {

ListNode prev = null;

ListNode curr = head;

while (curr != null) {

ListNode nextTemp = curr.next;

curr.next = prev;

prev = curr;

curr = nextTemp;

}

return prev;

}

public ListNode reverseList5(ListNode head) {

if (head == null || head.next == null) return head;

ListNode p = reverseList(head.next);

head.next.next = head;

head.next = null;

return p;

}

}