/**
1. bfs 按层,从左到右遍历,并为其加入指针
2. 处理当前层时, 我们可以认为上一层的平行指针已经建立完了, 可以直接遍历一层
*/

/*
// 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 || (root.left == null && root.right == null)) return root;
        return bfsWithSelf(root);
        // return bfsWithQueue(root);
    }

    public Node bfsWithSelf(Node root){

        for (Node p = root; p.left != null ; p = p.left){
            for (Node q = p; q != null ; q = q.next){
                q.left.next = q.right;
                if (q.next != null) q.right.next =  q.next.left;
            }
        }
        return root;
    }


    public Node bfsWithQueue(Node root){
        Queue<Node> queue = new LinkedList<>();
        queue.add(root);

        while (!queue.isEmpty()){
            int n = queue.size();
            for (int i = 0 ; i < n; i++){
                Node cur = queue.poll();
                if (i + 1 < n) cur.next = queue.peek();
                if (cur.left != null) queue.add(cur.left);
                if (cur.right != null) queue.add(cur.right);
            }
        }
        return root;
    }
}