116. Populating Next Right Pointers in Each Node/ 117. Populating Next Right Pointers in Each Node II

/*
// Definition for a Node.
class Node {
    public int val;
    public Node left;
    public Node right;
    public Node next;

    public Node() {}

    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) {
            return root;
        }
        root.left.next = root.right;
        if (root.next != null) {
            root.right.next = root.next.left;
        }
        
        connect(root.left);
        connect(root.right);
        return root;
    }
}

/*
// Definition for a Node.
class Node {
    public int val;
    public Node left;
    public Node right;
    public Node next;

    public Node() {}

    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;
        }
        Deque<Node> queue = new ArrayDeque<>();
        queue.offerLast(root);
        while (!queue.isEmpty()) {
            Node prev = null;
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                Node cur = queue.pollFirst();
                if (prev != null) {
                    prev.next = cur;
                }
                prev = cur;
                if (cur.left != null) {
                    queue.offerLast(prev.left);
                }
                if (cur.right != null) {
                    queue.offerLast(prev.right);
                }
            }
        }
        return root;
    }
}