public static void main(String[] args) {
int[] A = new int[] {1, 2, 3, 4};
ListNode head = ListNode.buildList(A);
ReverseLinkedList rll = new ReverseLinkedList();
ListNode ans = rll.reverseListRec(head);
ListNode.printNode(ans);
}
/**
* My first implementation of iteration
*
* @param head
* @return
*/
public ListNode reverseListIte1st(ListNode head) {
if (head == null) {
return head;
}
ListNode tail = head;
ListNode pre = new ListNode(0);
ListNode post;
while (head != null) {
post = head.next;
head.next = pre;
pre = head;
head = post;
}
tail.next = null;
return pre;
}
public ListNode reverseList(ListNode head) {
ListNode newHead = null;
while (head != null) {
ListNode next = head.next;
head.next = newHead;
newHead = head;
head = next;
}
return newHead;
}
private ListNode helper(ListNode head) {
if (head == null) {
return head;
}
if (head.next == null) {
return head;
}
ListNode newHead = helper(head.next);
head.next.next = head;
head.next = null;
return newHead;
}