[leetcode]Populating Next Right Pointers in Each Node II

题目大意：

Follow up for problem "Populating Next Right Pointers in Each Node".

What if the given tree could be any binary tree? Would your previous solution still work?

Note:

• You may only use constant extra space.

For example,
Given the following binary tree,

         1       /  \      2    3     / \    \    4   5    7

After calling your function, the tree should look like:

         1 -> NULL       /  \      2 -> 3 -> NULL     / \    \    4-> 5 -> 7 -> NULL

题目分析：

跟之前的Populating Next Right Pointers in Each Node有些类似，不过题目前提是一棵普通二叉树，回忆之前的Populating Next Right Pointers in Each Node的解法：

针对此题：可以先把第一层的next置为NULL，从第一层开始循环，每次循环是将：下一层连接起来（本层已经在每次循环前连接好了，只需要按照链表方式遍历本层节点，可以顺势把下一层连接起来，不过每次循环要找到下一层的起始位置）。