Reverse a singly linked list.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if(head == null || head.next == null)
            return head;

        ListNode prev = head;
        ListNode curr = head.next;
        ListNode next = head.next.next;
        head.next = null;

        while(next != null) {
            curr.next = prev;
            prev = curr;
            curr = next;
            next = next.next;
        }

        curr.next = prev;

        return curr;
    }
}