层序遍历，有点类似这道题。
104. 二叉树的最大深度

/*
// Definition for a Node.
class Node {public int val;public Node left;public Node right;public Node next;public Node() {}public Node(int _val) {val = _val;}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 null;root.next = null;Queue<Node> q = new LinkedList<>();q.offer(root);while (!q.isEmpty()) {int size = q.size();while (size > 0) {Node node = q.poll();if (node.left != null) q.offer(node.left);if (node.right != null) q.offer(node.right);--size;if (size > 0) node.next = q.peek();else node.next = null;}}return root;}
}