src/main/java/com/fishercoder/solutions/_117.java

package com.fishercoder.solutions;

public class _117 {

    public static class Node {
        public int val;
        public Node left;
        public Node right;
        public Node next;

        public Node(int x) {
            this.val = x;
        }
    }

    public static class Solution1 {
        /**
         * credit: https://discuss.leetcode.com/topic/1106/o-1-space-o-n-complexity-iterative-solution
         * O(1) space, based on level order traversal
         */
        public Node connect(Node root) {

            Node head = null; //head of the next level
            Node prev = null; //the leading node on the next level
            Node cur = root;  //current node of current level

            while (cur != null) {

                while (cur != null) { //iterate on the current level
                    //left child
                    if (cur.left != null) {
                        if (prev != null) {
                            prev.next = cur.left;
                        } else {
                            head = cur.left;
                        }
                        prev = cur.left;
                    }
                    //right child
                    if (cur.right != null) {
                        if (prev != null) {
                            prev.next = cur.right;
                        } else {
                            head = cur.right;
                        }
                        prev = cur.right;
                    }
                    //move to next node
                    cur = cur.next;
                }

                //move to next level
                cur = head;
                head = null;
                prev = null;
            }
            return root;
        }
    }
}