117.java

/*
// 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 root;
        Queue<Node> queue = new LinkedList<>();
        queue.add(root);
        while (queue.size() > 0) {
            int size = queue.size();
            Node prev = null;
            while (size > 0) {
                size--;
                Node node = queue.poll();
                if (prev != null) prev.next = node;
                prev = node;
                if (node.left != null) queue.add(node.left);
                if (node.right != null) queue.add(node.right);
            }
        }
        return root;
    }
}