Reverse Linked List

Java

class Solution {
		public ListNode reverseList(ListNode head) {
			  if (head == null || head.next == null) return head;
		    ListNode dummyNode = new ListNode(-1);
			  while (head != null) {
				//copy head.val / head.next to temp（保存head 的值 → temp为oldHead）
		    ListNode temp = head; 
				//copy head.next.val and head.next.next to head (head 移动）
		    head = head.next; 
				//将temp指向dummyNode指向的位置,也就是null (oldHead 指向 null)
				temp.next = dummyNode.next; 
				//将 dummyNode 指向 temp;
				dummyNode.next = temp; 
			}
				return dummyNode.next;
		}
}