#116填充每个节点的下一个右侧节点指针

import java.util.*;

class Node {
    int val;
    Node left;
    Node right;
    Node next;

    Node(int val) {
        this.val = val;
    }
}

class Solution {
    public Node connect(Node root) {
        if (root == null) return null;
        Deque<Node> queue = new ArrayDeque<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            Node previous = null;
            for (int size = queue.size(); size > 0; size--) {
                Node node = queue.poll();
                if (previous != null) previous.next = node;
                previous = node;
                if (node.left != null) queue.offer(node.left);
                if (node.right != null) queue.offer(node.right);
            }
            previous.next = null;
        }
        return root;
    }
}