flattenBfs created by smakar20 - https://repl.it/@smakar20/flattenBfs

flattenBfs.js

//Given a binary tree, flatten it to a linked list levelwise.

class Tree{
  constructor(val){
    this.val = val
    this.left = null
    this.right = null
  }
  
  insertLeft(val){
    this.left = new Tree(val)
    return this.left
  }
  
  insertRight(val){
    this.right = new Tree(val)
    return this.right
  }
}

class Node{
  constructor(val){
    this.val = val
    this.next = null
  }
}

function flattenBfs(root){
  if(!root) return
  var head = null
  var tail = null,
  arr = [root, '--']
  for(var node of arr){
    if(node == '--' && head)
    {
      print(head)
      console.log('-----')
      head = null
      tail = null
      arr.push('--')
    }else{
      if(!tail){
        tail = new Node(node.val)
        head = tail
      }else{
        tail.next = new Node(node.val)
        tail = tail.next
      }
    }
    
    if(node.left){
      arr.push(node.left)
    }
    if(node.right){
      arr.push(node.right)
    }
  }
}

function print(head){
  while(head){
    console.log(head.val)
    head = head.next
  }
}

// ----- test ------
var root = new Tree(1)
var left = root.insertLeft(2)
var right = root.insertRight(3)
left.insertLeft(4)
left.insertRight(5)
right.insertLeft(6)
right.insertRight(7)
flattenBfs(root)
//----output-----
/*
1
-----
2
3
-----
4
5
6
7
-----
*/
//---------------------