116.populating-next-right-pointers-in-each-node.java

/*
 * @lc app=leetcode id=116 lang=java
 *
 * [116] Populating Next Right Pointers in Each Node
 *
 * https://leetcode.com/problems/populating-next-right-pointers-in-each-node/description/
 *
 * algorithms
 * Medium (39.05%)
 * Likes:    1144
 * Dislikes: 97
 * Total Accepted:    263.6K
 * Total Submissions: 675.2K
 * Testcase Example:  '{"$id":"1","left":{"$id":"2","left":{"$id":"3","left":null,"next":null,"right":null,"val":4},"next":null,"right":{"$id":"4","left":null,"next":null,"right":null,"val":5},"val":2},"next":null,"right":{"$id":"5","left":{"$id":"6","left":null,"next":null,"right":null,"val":6},"next":null,"right":{"$id":"7","left":null,"next":null,"right":null,"val":7},"val":3},"val":1}'
 *
 * You are given a perfect binary tree where all leaves are on the same level,
 * and every parent has two children. The binary tree has the following
 * definition:
 *
 *
 * struct Node {
 * ⁠ int val;
 * ⁠ Node *left;
 * ⁠ Node *right;
 * ⁠ Node *next;
 * }
 *
 *
 * Populate each next pointer to point to its next right node. If there is no
 * next right node, the next pointer should be set to NULL.
 *
 * Initially, all next pointers are set to NULL.
 *
 *
 *
 * Example:
 *
 *
 *
 *
 * Input:
 * {"$id":"1","left":{"$id":"2","left":{"$id":"3","left":null,"next":null,"right":null,"val":4},"next":null,"right":{"$id":"4","left":null,"next":null,"right":null,"val":5},"val":2},"next":null,"right":{"$id":"5","left":{"$id":"6","left":null,"next":null,"right":null,"val":6},"next":null,"right":{"$id":"7","left":null,"next":null,"right":null,"val":7},"val":3},"val":1}
 *
 * Output:
 * {"$id":"1","left":{"$id":"2","left":{"$id":"3","left":null,"next":{"$id":"4","left":null,"next":{"$id":"5","left":null,"next":{"$id":"6","left":null,"next":null,"right":null,"val":7},"right":null,"val":6},"right":null,"val":5},"right":null,"val":4},"next":{"$id":"7","left":{"$ref":"5"},"next":null,"right":{"$ref":"6"},"val":3},"right":{"$ref":"4"},"val":2},"next":null,"right":{"$ref":"7"},"val":1}
 *
 * Explanation: Given the above perfect binary tree (Figure A), your function
 * should populate each next pointer to point to its next right node, just like
 * in Figure B.
 *
 *
 *
 *
 * Note:
 *
 *
 * You may only use constant extra space.
 * Recursive approach is fine, implicit stack space does not count as extra
 * space for this problem.
 *
 *
 */
/*
// 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;

    Queue<Node> queue = new LinkedList<>();
    queue.offer(root);

    while (!queue.isEmpty()) {
      int size = queue.size();
      Node prev = null;
      for (int i = 0; i < size; i++) {
        Node node = queue.poll();
        if (prev != null) {
          prev.next = node;
        }
        if (node.left != null) {
          queue.offer(node.left);
        }
        if (node.right != null) {
          queue.offer(node.right);
        }
        prev = node;
      }
      prev.next = null;
    }

    return root;
  }
}