Leetcode Problem 107 - Binary Tree Level Order Traversal II.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Problem Statement.\n",
    "\n",
    "Given the root of a binary tree, return the bottom-up level order traversal of its nodes' values. (i.e., from left to right, level by level from leaf to root).\n",
    "\n",
    " \n",
    "\n",
    "Example 1:\n",
    "\n",
    "Input: root = [3,9,20,null,null,15,7]\n",
    "Output: [[15,7],[9,20],[3]]\n",
    "\n",
    "Example 2:\n",
    "\n",
    "Input: root = [1]\n",
    "Output: [[1]]\n",
    "\n",
    "Example 3:\n",
    "\n",
    "Input: root = []\n",
    "Output: []\n",
    "\n",
    " \n",
    "\n",
    "Constraints:\n",
    "\n",
    "    The number of nodes in the tree is in the range [0, 2000].\n",
    "    -1000 <= Node.val <= 1000"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# DFS and list reverse - O(N) runtime, O(N) space"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [],
   "source": [
    "from typing import List\n",
    "\n",
    "# Definition for a binary tree node.\n",
    "# class TreeNode:\n",
    "#     def __init__(self, val=0, left=None, right=None):\n",
    "#         self.val = val\n",
    "#         self.left = left\n",
    "#         self.right = right\n",
    "\n",
    "class Solution:\n",
    "    def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:\n",
    "        levels = []\n",
    "        if not root: return levels\n",
    "        \n",
    "        def helper(node, level):\n",
    "            # start the current level\n",
    "            if len(levels) == level: levels.append([])\n",
    "\n",
    "            # append the current node value\n",
    "            levels[level].append(node.val)\n",
    "\n",
    "            # process child nodes for the next level\n",
    "            if node.left:\n",
    "                helper(node.left, level + 1)\n",
    "            if node.right:\n",
    "                helper(node.right, level + 1)\n",
    "            \n",
    "        helper(root, 0)\n",
    "        return levels[::-1]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# BFS and Deque - O(N) runtime, O(N) space"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [],
   "source": [
    "from typing import List\n",
    "from collections import deque\n",
    "\n",
    "# Definition for a binary tree node.\n",
    "# class TreeNode:\n",
    "#     def __init__(self, val=0, left=None, right=None):\n",
    "#         self.val = val\n",
    "#         self.left = left\n",
    "#         self.right = right\n",
    "\n",
    "class Solution:\n",
    "    def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:\n",
    "        if not root: return []\n",
    "        result = deque()\n",
    "        queue = deque([root])\n",
    "        \n",
    "        while queue:\n",
    "            n = len(queue)\n",
    "            currLevel = []\n",
    "            for _ in range(n):\n",
    "                node = queue.popleft()\n",
    "                currLevel.append(node.val)\n",
    "                for child in [node.left, node.right]:\n",
    "                    if child: queue.append(child)\n",
    "                        \n",
    "            result.appendleft(currLevel)\n",
    "            \n",
    "        return result"
   ]
  }
 ],
 "metadata": {
  "kernelspec": {
   "display_name": "Python 3",
   "language": "python",
   "name": "python3"
  },
  "language_info": {
   "codemirror_mode": {
    "name": "ipython",
    "version": 3
   },
   "file_extension": ".py",
   "mimetype": "text/x-python",
   "name": "python",
   "nbconvert_exporter": "python",
   "pygments_lexer": "ipython3",
   "version": "3.7.3"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 2
}