0206-reverse-linked-list.java

//Use three pointers and so you can change the next of the mid to the first one without losing the track of the original left.
//Iterative version
class Solution {

    public ListNode reverseList(ListNode head) {
        ListNode p = null;
        ListNode q = null;
        ListNode r = head;
        while (r != null) {
            p = q;
            q = r;
            r = r.next;
            q.next = p;
        }
        return q;
    }
}

//Recursive version
class Solution {

    public ListNode reverseList(ListNode head) {
        return rev(head, null);
    }

    public ListNode rev(ListNode node, ListNode pre) {
        if (node == null) return pre;
        ListNode temp = node.next;
        node.next = pre;
        return rev(temp, node);
    }
}