206. Reverse 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) {
        ListNode prev = null;
        ListNode curr = head;
        while (curr != null) {
            ListNode oldNext = curr.next;	// 1. stores the old next
            curr.next = prev;				// 2. the current points to the previous
            prev = curr;					// 3. the current becomes a new previous
            curr = oldNext;					// 4. the current moves on
        }
        return prev;
    }
}