_116.java

package com.fishercoder.solutions;

import com.fishercoder.common.classes.TreeLinkNode;

/** 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,
      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 */

public class _116 {
    //credit: https://discuss.leetcode.com/topic/1106/o-1-space-o-n-complexity-iterative-solution
    //based on level order traversal
    public static void connect(TreeLinkNode root) {

        TreeLinkNode head = null; //head of the next level
        TreeLinkNode prev = null; //the leading node on the next level
        TreeLinkNode curr = root;  //current node of current level

        while (curr != null) {
            while (curr != null) { //iterate on the current level
                //left child
                if (curr.left != null) {
                    if (prev != null) {
                        prev.next = curr.left;
                    } else {
                        head = curr.left;
                    }
                    prev = curr.left;
                }
                //right child
                if (curr.right != null) {
                    if (prev != null) {
                        prev.next = curr.right;
                    } else {
                        head = curr.right;
                    }
                    prev = curr.right;
                }
                //move to next node
                curr = curr.next;
            }
            //move to next level
            curr = head;
            head = null;
            prev = null;
        }

    }

    public static void main(String...args){
        TreeLinkNode root = new TreeLinkNode(1);
        root.left = new TreeLinkNode(2);
        root.right = new TreeLinkNode(3);
        root.left.left = new TreeLinkNode(4);
        root.left.right = new TreeLinkNode(5);
        root.right.right = new TreeLinkNode(7);
        connect(root);
    }
}