206. reverse linked list

 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class  {
    public ListNode reverseList(ListNode head) {
        ListNode prev = null;
        ListNode curr = head;
        while (curr != null) {
          ListNode temp = curr.next;
            curr.next = prev;
            prev = curr;
            curr = temp;
        }
        head = prev;
        return head;
        
    }
}