[LeetCode] Populating Next Right Pointers in Each Node II

Problem:

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

Solution:O(n)

public class Solution {  
    public void connect(TreeLinkNode root) {  
        if (root == null) 
            return;  
        if (root.left != null) {  
            if (root.right != null) {  
                root.left.next = root.right;  
            }  
            else {  
                TreeLinkNode p = root.next;  
                while (p != null && p.left == null && p.right == null)  
                    p = p.next;  
                if (p != null)  
                    root.left.next = (p.left == null ? p.right : p.left);  
            }  
        }  
          
        if (root.right != null) {  
            TreeLinkNode p = root.next;  
            while (p != null && p.left == null && p.right == null)  
                p = p.next;  
            if (p != null)  
                root.right.next = (p.left == null ? p.right : p.left);  
        }  
        connect(root.right);      
        connect(root.left);  
    }  
} 

Key Concept: Recursive and connect right first