Let Code Day83 Starting from Zero "102. Binary Tree Level Order Traversal"

Overview

It seems that coding tests are conducted overseas in interviews with engineers, and in many cases, the main thing is to implement specific functions and classes according to the theme.

Apparently, many engineers take measures on the site called LetCode.

It is a site that trains the algorithmic power that can withstand the coding test that is being done in the early story, and it is an inevitable path for those who want to build a career at an overseas tech company.

I wrote it in a big way, but I have no plans to have such an interview at the moment.

However, as an IT engineer, it would be better to have the same level of algorithm power as a person, so I would like to solve the problem irregularly and write down the method I thought at that time as a memo.

Leetcode

I'm solving it with Python3.

Leet Code Table of Contents Starting from Zero

Last time Leet Code Day 82 "392. Is Subsequence" starting from zero

Twitter I'm doing it.

** Technical Blog Started! !! ** ** I think the technology will write about LetCode, Django, Nuxt, and so on. ** This is faster to update **, so please bookmark it!

problem

102. Binary Tree Level Order Traversal The difficulty level is Medium. It is an excerpt from the problem collection as before.

The problem is, given a binary tree, design an algorithm that returns a list of equivalents next to the value hierarchy of that node. (That is, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],
    3
   / \
  9  20
    /  \
   15   7
return its level order traversal as:
[
  [3],
  [9,20],
  [15,7]
]

An example is like this. I hope you understand what I mean.

solution

Solved by DFS.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def levelOrder(self, root: TreeNode) -> List[List[int]]:
        ans,level = [],0
        self.dfs(root,level,ans)
        return ans
    
    def dfs(self,root,level,ans):
        if not root:
            return
        if len(ans) < level+1:
            ans.append([])
        ans[level].append(root.val)
        self.dfs(root.left,level+1,ans)
        self.dfs(root.right,level+1,ans)
# Runtime: 32 ms, faster than 86.02% of Python3 online submissions for Binary Tree Level Order Traversal.
# Memory Usage: 14.2 MB, less than 43.09% of Python3 online submissions for Binary Tree Level Order Traversal.

It's a simple DFS. Since it is decided to return it as a list, if you increase the length of the list, you can cover the elements of the hierarchy well.

This time I solved it with a depth-first search that I am accustomed to, but there are examples of solving with BFS and answers using cues in discuss, so it seems that an individual's favorite solution will appear. That's a problem.

So that's it for this time. Thank you for your hard work.

Recommended Posts

Let Code Day83 Starting from Zero "102. Binary Tree Level Order Traversal"
Let Code Day 44 "543. Diameter of Binary Tree" starting from zero
Let Code Day 26 starting from scratch "94. Binary Tree Inorder Traversal"
Let Code Day 23 "226. Invert Binary Tree" starting from scratch
Let Code Day 40 Starting from Zero "114. Flatten Binary Tree to Linked List"
Let Code Day12 Starting from Zero "617. Merge Two Binary Trees"
Let Code Day58 Starting from Zero "20. Valid Parentheses"
Let Code Day 27 "101. Symmetric Tree" starting from scratch
Let Code Day16 Starting from Zero "344. Reverse String"
Let Code Day49 starting from zero "1323. Maximum 69 Number"
Let Code Day89 "62. Unique Paths" Starting from Zero
Let Code Day 55 "22. Generate Parentheses" Starting from Zero
Let Code Day18 starting from zero "53. Maximum Subarray"
Let Code Day 13 "338. Counting Bits" Starting from Zero
Let Code Day 82 "392. Is Subsequence" Starting from Zero
Let Code Day51 Starting from Zero "647. Palindromic Substrings"
Let Code Day14 starting from zero "136. Single Number"
Let Code Day 9 "701. Insert into a Binary Search Tree" starting from scratch
Let Code Day 43 "5. Longest Palindromic Substring" Starting from Zero
Let Code Day74 starting from zero "12. Integer to Roman"
Let Code Day 42 "2. Add Two Numbers" Starting from Zero
Let Code Day57 Starting from Zero "35. Search Insert Position"
Let Code Day47 Starting from Zero "14. Longest Common Prefix"
Let Code Day78 Starting from Zero "206. Reverse Linked List"
Let Code Day45 Starting from Zero "1379. Find a Corresponding Node of a Binary Tree in a Clone of That Tree"
Let Code Day1 Starting from Zero "1389. Create Target Array in the Given Order"
Let Code Day 64 starting from zero "287. Find the Duplicate Number"
Let Code Day 84 starting from zero "142. Linked List Cycle II"
Let Code Day24 Starting from Zero "21. Merge Two Sorted Lists"
Let Code Day2 Starting from Zero "1108. Defanging an IP Address"
Let Code Day70 Starting from Zero "295. Find Median from Data Stream"
Let Code Day 38 starting from scratch "208. Implement Trie (Prefix Tree)"
Let Code Day81 "347. Top K Frequent Elements" Starting from Zero
Let Code Day48 Starting from Zero "26. Remove Duplicates from Sorted Array"
Let Code Day67 Starting from Zero "1486. XOR Operation in an Array"
Let Code Day56 Starting from Zero "5453. Running Sum of 1d Array"
Let Code Day92 Starting from Zero "4. Median of Two Sorted Arrays"
Let Code Day5 starting from zero "1266. Minimum Time Visiting All Points"
Let Code Day 35 "160. Intersection of Two Linked Lists" Starting from Zero
Let Code Day54 Starting from Zero "1290. Convert Binary Number in a Linked List to Integer"
Let Code Day 41 "394. Decode String" starting from scratch
Let Code Day 25 "70. Climbing Stairs" starting from scratch
Let Code Day 34 starting from scratch "118. Pascal's Triangle"
Let Code Day20 starting from scratch "134. Gas Station"
Let Code Day 88 "139. Word Break" starting from scratch
Let Code Day 28 "198. House Robber" starting from scratch
Let Code Day 39 "494. Target Sum" starting from scratch
Let Code Day 36 "155. Min Stack" starting from scratch
Let Code Day 17 "169. Majority Element" starting from scratch
Let Code Day 33 "1. Two Sum" starting from scratch
Let Code Day 91 "153. Find Minimum in Rotated Sorted Array" starting from zero
Let Code Day59 starting from zero "1221. Split a String in Balanced Strings"
Let Code Day 11 Starting from Zero "1315. Sum of Nodes with Even-Valued Grandparent"
Let Code Day6 Starting from Zero "1342. Number of Steps to Reduce a Number to Zero"
Let Code Day8 starting from scratch "1302. Deepest Leaves Sum"
Let Code Day 22 starting from scratch "141. Linked List Cycle"
Let Code Day73 Starting from Zero "1491. Average Salary Excluding the Minimum and Maximum Salary"
Let Code Day10 Starting from Zero "1431. Kids With the Greatest Number of Candies"
Let Code Day 30 starting from scratch "234. Palindrome Linked List"
Let Code Day21 Starting from Zero "448. Find All Numbers Disappeared in an Array"
Let Code Day68 starting from scratch "709. To Lower Case"