广度优先搜索

class Solution {
    public Node connect(Node root) {

        Queue<Node> queue = new LinkedList<>();

        if (root == null){
            return root;
        }

        queue.add(root);

        while (!queue.isEmpty()){

            int size = queue.size();

            for (int i = 0; i < size; i++) {

                Node temp = queue.poll();

                /**
                 * 每一层最后一个节点的next指向null，其他节点指向下一个出队的节点
                 */
                if (i == size - 1){
                    temp.next = null;
                }
                else {
                    temp.next = queue.peek();
                }

                if (temp.left != null){
                    queue.add(temp.left);
                }

                if (temp.right != null){
                    queue.add(temp.right);
                }
            }
        }

        return root;
    }
}

/**
 * 时间复杂度 O(n)
 * 空间复杂度 O(n)
 */

https://leetcode-cn.com/problems/populating-next-right-pointers-in-each-node/