116. Populating Next Right Pointers in Each Node java solutions

Given a binary tree

    struct TreeLinkNode {
      TreeLinkNode *left;
      TreeLinkNode *right;
      TreeLinkNode *next;
    }

Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to NULL.

Initially, all next pointers are set to NULL.

Note:

You may only use constant extra space.
You may assume that it is a perfect binary tree (ie, all leaves are at the same level, and every parent has two children).

For example,
Given the following perfect binary tree,

After calling your function, the tree should look like:

         1 -> NULL
       /  
      2 -> 3 -> NULL
     /   / 
    4->5->6->7 -> NULL

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 1 /**
 2  * Definition for binary tree with next pointer.
 3  * public class TreeLinkNode {
 4  *     int val;
 5  *     TreeLinkNode left, right, next;
 6  *     TreeLinkNode(int x) { val = x; }
 7  * }
 8  */
 9 public class Solution {
10     public void connect(TreeLinkNode root) {
11         if(root == null) return;
12         TreeLinkNode cur = root;//使用cur来逐层遍历
13         while(root.left != null){
14             cur = root;
15             while(cur != null){
16                 cur.left.next = cur.right;
17                 if(cur.next != null)
18                     cur.right.next = cur.next.left;
19                 cur = cur.next;
20             }
21             root = root.left;
22         }
23     }
24 }

本题使用栈或者队列也是可以做的。但是这样的话就超出题目

You may only use constant extra space. 的要求。

如果不是完全二叉树又该如何解决呢？