問題：

給一個單向連結串列，把它從頭到尾反轉過來。比如： a -> b -> c ->d 反過來就是 d -> c -> b -> a 。

這裡講解兩種方法：

第一種方法就是把每個Node按照順序存入到一個stack裡面，這樣，最後面一個就在最上面了。然後，把每一個再取出來，這樣順序就換過來了。

public static Node reverse(Node head) {
	Stack<Node> stack = new Stack<Node>();
	
	// put all the nodes into the stack
	while (head != null) {
		stack.add(head);
		head = head.next();
	}
	
	//reverse the linked list
	Node current = stack.pop();
	head = current;
	while (stack.empty() != true) {
		Node next = stack.pop();
		//set the pointer to null, so the last node will not point to the first node.
		next.setNext(null);
		current.setNext(next);
		current = next;
	}
	
	return head;	
}
 

public static Node reverse(Node head) {
	Node previous = null;

	while (head != null) {
		Node nextNode = head.next();
		head.setNext(previous);
		previous = head;
		head = nextNode;
	}
		
	return previous;	
}