今天发现自己对于数据结构有点陌生了，最近几年主要精力放在语言层次上，这些基本的东西反而有些陌生了，温故而知新，决定花些时间把这些东西整理下。本文不定期更新。

- 树的层次遍历

输入

        4
      /   \
     6     9
    / \     \
   7   8     12

要求打印出 4->6->9->7->8->12

public void PrintByLevel() {
        LinkedList<Tree> l = new LinkedList<Tree>();
        l.add(this);
        while (!l.isEmpty()) {
            Tree tmp = l.pollFirst();
            System.out.println(tmp.getValue());
            if (null != tmp.getLChild()) {
                l.add(tmp.getLChild());
            }
            if (null != tmp.getRChild()) {
                l.add(tmp.getRChild());
            }
        }
    }

• 链表的递归和非递归反转
  递归反转

// 递归，在反转当前节点之前先反转后续节点

public static ListNode Reverse(ListNode head) {
        if (null == head || null == head.next) {
            return head;
        }
        ListNode reversedHead = Reverse(head.next);
        head.next.next = head;
        head.next = null;
        return reversedHead;
    }

非递归反转

public static ListNode NoneRecursiveReverse(ListNode head) {
    ListNode retval = null;
    ListNode node = head;
    ListNode preNode = null;
    while ( node != null ){
        // get the next node, and save it at pNext
        ListNode nextNode = node.next;
        // if the next node is null, the current is the end of original
        // list, and it's the head of the reversed list
        if ( nextNode == null) {
            retval = node;
        }
        // reverse the linkage between nodes
        node.next = preNode;
        preNode = node;
        node = nextNode;
    }
    return retval;
}