思路：遍歷整個鏈表，維護兩個節點pre和next，node.next每次都指向pre，然後這三個點同時向後移動。
public static Node reverseList(Node node){
Node pre = null;
Node next = null;
while(node != null){
next = node.next;
node.next = pre;
pre = node;
node = next;
}
}