116. Populating Next Right Pointers in Each Node

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 toNULL.

Initially, all next pointers are set toNULL.

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,

         1
       /  \
      2    3
     / \  / \
    4  5  6  7

After calling your function, the tree should look like:

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

Solution:

keep track of parent node,

1. parent.left.next = parent.right
2. parent.right.next = parent.next.left (parent.next != null)

public class Solution {
    public void connect(TreeLinkNode root) {
        if(root == null){
            return;
        } 

        TreeLinkNode curr = root;
        TreeLinkNode prev = null;

        while(curr.left != null){
            prev = curr;
            while(prev != null){
                prev.left.next = prev.right;
                if(prev.next != null){
                    prev.right.next = prev.next.left;
                }
                prev = prev.next;
            }
            curr = curr.left;
        }

    }