Populating Next Right Pointers in Each Node | Leetcode

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

---

Solution: Terminate the linked list of the root node first. Maintain two queues, first queue (curLevel) is the current level of the tree, second queue (nextLevel) is the next level of the tree. For each level, maintain a previous node (prevNode). Link the previous node to the current node. When you are done iterating through the entire tree, the linking is done.

/**
 * Definition for binary tree with next pointer.
 * public class TreeLinkNode {
 *     int val;
 *     TreeLinkNode left, right, next;
 *     TreeLinkNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void connect(TreeLinkNode root) {
        if (root == null) return;
        root.next = null;
        
        // Iterate through each level of the tree.
        Queue<TreeLinkNode> curLevel = new LinkedList<>();
        Queue<TreeLinkNode> nextLevel = new LinkedList<>();
        
        if (root.left != null) curLevel.add(root.left);
        if (root.right != null) curLevel.add(root.right);
        
        while (!curLevel.isEmpty()) {
            TreeLinkNode prevNode = null;
            do {
                TreeLinkNode curNode = curLevel.remove();
                if (curNode.left != null) nextLevel.add(curNode.left);
                if (curNode.right != null) nextLevel.add(curNode.right);
                if (prevNode != null) prevNode.next = curNode;
                prevNode = curNode;
            } while (!curLevel.isEmpty());
            
            // Terminate the current level linked list.
            if (prevNode != null) prevNode.next = null;
            
            // Swap the curLevel and nextLevel queues.
            Queue<TreeLinkNode> temp = curLevel;
            curLevel = nextLevel;
            nextLevel = temp;
        }
    }
}