{
 "cells": [
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   "cell_type": "markdown",
   "source": [
    "Given a binary tree, return the _level order_ traversal of its nodes' values.\n",
    "(ie, from left to right, level by level).\n",
    "\n",
    "For example:\n",
    "Given binary tree `[3,9,20,null,null,15,7]`,\n",
    "\n",
    "\n",
    "\n",
    "        3\n",
    "       / \\\n",
    "      9  20\n",
    "        /  \\\n",
    "       15   7\n",
    "\n",
    "\n",
    "return its level order traversal as:\n",
    "\n",
    "\n",
    "\n",
    "    [\n",
    "      [3],\n",
    "      [9,20],\n",
    "      [15,7]\n",
    "    ]"
   ],
   "metadata": {}
  },
  {
   "outputs": [
    {
     "output_type": "execute_result",
     "data": {
      "text/plain": "level_order (generic function with 1 method)"
     },
     "metadata": {},
     "execution_count": 1
    }
   ],
   "cell_type": "code",
   "source": [
    "# @lc code=start\n",
    "using LeetCode\n",
    "\n",
    "function level_order(root::Union{Nothing, TreeNode{Int}})\n",
    "    isnothing(root) && return Int[]\n",
    "    q = TreeNode{Int}[]\n",
    "    q2 = TreeNode{Int}[]\n",
    "    res = Vector{Int}[]\n",
    "    push!(q, root)\n",
    "    while !isempty(q)\n",
    "        push!(res, Int[])\n",
    "        while !isempty(q)\n",
    "            fst = popfirst!(q)\n",
    "            push!(res[end], fst.val)\n",
    "            !isnothing(fst.left) && push!(q2, fst.left)\n",
    "            !isnothing(fst.right) && push!(q2, fst.right)\n",
    "        end\n",
    "        q, q2 = q2, q\n",
    "    end\n",
    "    return res\n",
    "end\n",
    "# @lc code=end"
   ],
   "metadata": {},
   "execution_count": 1
  },
  {
   "cell_type": "markdown",
   "source": [
    "---\n",
    "\n",
    "*This notebook was generated using [Literate.jl](https://github.com/fredrikekre/Literate.jl).*"
   ],
   "metadata": {}
  }
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