[LeetCode 117] Populating Next Right Pointers in Each Node II (Medium)

Given a binary tree

struct Node {
  int val;
  Node *left;
  Node *right;
  Node *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.

Example:

image

**Input:** {"$id":"1","left":{"$id":"2","left":{"$id":"3","left":null,"next":null,"right":null,"val":4},"next":null,"right":{"$id":"4","left":null,"next":null,"right":null,"val":5},"val":2},"next":null,"right":{"$id":"5","left":null,"next":null,"right":{"$id":"6","left":null,"next":null,"right":null,"val":7},"val":3},"val":1}

**Output:** {"$id":"1","left":{"$id":"2","left":{"$id":"3","left":null,"next":{"$id":"4","left":null,"next":{"$id":"5","left":null,"next":null,"right":null,"val":7},"right":null,"val":5},"right":null,"val":4},"next":{"$id":"6","left":null,"next":null,"right":{"$ref":"5"},"val":3},"right":{"$ref":"4"},"val":2},"next":null,"right":{"$ref":"6"},"val":1}

**Explanation:** Given the above binary tree (Figure A), your function should populate each next pointer to point to its next right node, just like in Figure B.

Note:

• You may only use constant extra space.
• Recursive approach is fine, implicit stack space does not count as extra space for this problem.

SOLUTION

1. 双重循环，内循环遍历每一层并connect all subtrees of current layer node，外循环 traverse vertically, move to next layer。
2. 每一层遍历时需要记录ChildHead， 横向移动时记录prev node。
   • ChildHead用于每次每一层起始的时候，同时标记第一个prev node。

• 如果 ChildHead不为空，那么说明prev node已经存在，只需要将prev node的next设置成 parent.left or parent.right

/*
// Definition for a Node.
class Node {
    public int val;
    public Node left;
    public Node right;
    public Node next;

    public Node() {}

    public Node(int _val,Node _left,Node _right,Node _next) {
        val = _val;
        left = _left;
        right = _right;
        next = _next;
    }
};
*/
class Solution {
    public Node connect(Node root) {
        if (root == null) {
            return root;
        }
        
        Node parent = root;
        Node childHead = null;
        Node prev = null;
        
        while (parent != null) {
            // traverse current layer and connect the subtrees of current layer nodes
            while (parent != null) {
                if (parent.left != null) {
                    if (childHead == null) {
                        childHead = parent.left;
                        prev = parent.left;
                    } else {
                        prev.next = parent.left;
                        prev = prev.next;
                    }
                }
                
                if (parent.right != null) {
                    if (childHead == null) {
                        childHead = parent.right;
                        prev = parent.right;
                    } else {
                        prev.next = parent.right;
                        prev = prev.next;
                    }
                }
                
                parent = parent.next;
            }
            
            // traverse vertically, move to next layer
            parent = childHead;
            childHead = null;
            prev = null;
        }
        
        return root;
    }
}