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【Leetcode】python – [206] Reverse Linked List 個人解法筆記 (內含範例程式碼) 內含 Linked List reverse 反轉操作 part 3 (for 新手教學)

題目出處

206. Reverse Linked List

難度

easy

題目分類

Linked List, Recursion

Linked List reverse 反轉操作 part 3

這邊我們要來研究 Linked list 的 反轉操作,
一樣的,這操作硬要說也不難,但只要思緒不夠清楚,
一定是轉了半天轉不出來XDD 最後絕望放棄XD

我們試著來把概念講的簡單一點吧!

試試看講簡單一點

做反轉動作來說,最簡單我們都需要「三個紀錄點」,
分別代表「過去」、「現在」、「未來」,
每一個記錄點都非常重要,
我們一定要注意「不能在過程中失去任何重要紀錄點,一旦被蓋掉就再也找不回來了!

因此我們基本的策略就有了下圖,1 ~ 3,
我們移動的方式一定是:

  1. 過去 = 現在
  2. 現在 = 未來
  3. 未來 = 更未來

到這邊,「務必務必謹守上述這個原則」,
再來我們再來討論反轉,什麼時候適合做反轉呢?
就是在做上述的「移動 1~3」之前,所以「更改」反轉的線,就是在第 0 步驟,

初始化

這裡有個小重點,不論如何,我們的 Linklist 最終「必須結束於 None」,
因此我們要先宣告個「新結束點」給他
那想當然,可以讓一開始 prev 擔任這個步驟。

而我們真正作為起頭的 head ,可以先給予他 cur (代表現在),
由於此時我們還不確定有沒有 head 存在,因此還不能直接給 head.next (會跳錯)

個人範例程式碼

記得上述所說的,我們需要先宣告 None 作為結尾

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        prev = None # must have a None end.
        if head: # init 
            cur = head
            head = head.next
            cur.next = prev # must have a None end.
        else: # special case
            return head

        # loop
        while head:   
            prev = cur
            cur = head  
            head = head.next
            cur.next = prev # reverse

        return cur 

Time Complexity

O(n)

Space Complexity

O(1)

算法說明

遍歷一次 head 是一定要的,問題是我們該怎麼移除,
移除的方法我們上面也提到過了,
只需要把「下個」替換成「下下個」就可以輕鬆解決,
只是要多考慮一些「不存在的邊際情況」,這邊我們下面會提。

corner case 特殊情況處理

一同整理在下方「邊際情況處理」

Boundary conditions/ Edge conditions 邊際情況處理

我們把題目稍微分析一下,我這裡大概把題目分類成於以下三種情況,

  1. [None]
  2. [1]
  3. [1,2,3] <- 正常情況

其中我們來看看 1.2. 我們該怎麼辦。

1. [None]

如果一開始 if head 就沒東西,我們就直接 return head

2. [1]

我們已經把 cur = head,而 while head 會直接判找不到東西,
因此回傳 cur,沒有問題。

Reference

⭐ Leetcode 解題紀錄 ⭐題型資料結構Python SolutionC++ SolutionNote
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103Binary Tree Zigzag Level Order TraversalBFS (分層)TreePython
107Binary Tree Level Order Traversal IIBFS (分層)TreePython
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127Word LadderBFS (分層), DFSGraphPython
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210Course Schedule IIBFS (拓樸)GraphPython
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[Lint] 431Connected Component in Undirected GraphBFS (連通塊)GraphPython 內含 BFS 模板 #重要題型
1091Shortest Path in Binary MatrixBFS (最短路徑)MatrixPython
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162Find Peak ElementBinary SerachPython
278First Bad VersionBinary SerachPython
658Find K Closest ElementsBinary SerachPython
704Binary SearchBinary SerachPython
852Peak Index in a Mountain ArrayBinary SerachPython
[Lint] 14First Position of TargetBinary SerachPython
[Lint] 140Fast PowerBinary SerachPython
[Lint] 447Search in a Big Sorted ArrayBinary SerachArrayPython
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22Generate ParenthesesDFSPython
79Word SearchDFSMatrixPython
126Word Ladder IIDFSPython
212Word Search IIDFSTreePython
290Word PatternDFSPython
[Lint] 829Word Pattern IIDFSPython
31Next PermutationDFS (排列)Python
46PermutationsDFS (排列)Python
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47Permutations IIDFS (排列)Python
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51N-QueensDFS (排列)Python
52N-Queens IIDFS (排列)Python
[Lint] 862Next Closest TimeDFS (排列)Python
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10Regular Expression MatchingDFS (組合)Python
77CombinationsDFS (組合)Python
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40Combination Sum IIDFS (組合)Python
216Combination Sum IIIDFS (組合)Python
377Combination Sum IVDFS (組合)Python
44Wildcard MatchingDFS (組合)Python
78SubsetsDFS (組合)Python
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131Palindrome PartitioningDFS (組合)Python
139Word BreakDFS (組合)Python
140Word Break IIDFS (組合)Python
[Lint] 90k Sum IIDFS (組合)Python
[Lint] 680Split StringDFS (組合)Python
173Binary Search Tree IteratorDFS (BST)BSTPython
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230Kth Smallest Element in a BSTDFS (BST)BSTPython
[Lint] 448Inorder Successor in BSTDFS (BST)BSTPython
[Lint] 900Closest Binary Search Tree ValueDFS (BST)BSTPython
[Lint] 901Closest Binary Search Tree Value IIDFS (BST)BSTPython
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208Implement Trie (Prefix Tree)DFS (Graph)TreePython
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226Invert Binary TreeDFS (Tree)TreePython
572Subtree of Another TreeDFS (Tree)TreePython
105Construct Binary Tree from Preorder and Inorder TraversalDFS (Tree)TreePython
112Path SumDFS (Tree)Python
113Path Sum IIDFS (Tree)TreePython
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236Lowest Common Ancestor of a Binary TreeDFS (Tree)TreePython
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257Binary Tree PathsDFS (Tree)TreePython
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[Lint] 578Lowest Common Ancestor IIIDFS (Tree)TreePython
[Lint] 596Minimum SubtreeDFS (Tree)TreePython
543Diameter of Binary TreeDFS (Tree)Python
144Binary Tree Preorder TraversalDFS (Tree)TreePython
內含 處理 Tree 樹問題的重點
145Binary Tree Postorder TraversalDFS (Tree)TreePython
114Flatten Binary Tree to Linked ListDFS (Tree)TreePython
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309Best Time to Buy and Sell Stock with CooldownDPPython
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91Decode WaysDP (一維)Python
55Jump GameDP (一維)Python
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213House Robber IIDP (一維)Python
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122Best Time to Buy and Sell Stock IIDP (一維)Python
300Longest Increasing SubsequenceDP (一維接龍, LIS)Python
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62Unique PathsDP (二維)Python C++
63Unique Paths IIDP (二維)Python C++
152Maximum Product SubarrayDP (二維)Python
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322Coin ChangeDP (背包問題)Python
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518Coin Change 2DP (背包問題)Python
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[Lint] 92BackpackDP (背包問題)Python
[Lint] 125Backpack IIDP (背包問題)Python
[Lint] 440Backpack IIIDP (背包問題)Python
[Lint] 562Backpack IVDP (背包問題)Python
[Lint] 563Backpack VDP (背包問題)Python
[Lint] 798Backpack VIIDP (背包問題)Python
[Lint] 799Backpack VIIIDP (背包問題)Python
[Lint] 800Backpack IXDP (背包問題)Python
[Lint] 801Backpack XDP (背包問題)Python
1143Longest Common SubsequenceDP (LCS)Python
494Target SumDP (Memoization)Python
2267Check if There Is a Valid Parentheses String PathDP (Memoization)Python
內含:Memoization 記憶化搜索筆記 / 292nd Weekly Contest
⭐ Hash 相關題型 ⭐
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73Set Matrix Zeroes HashPython C++ 內含 python while-else 用法說明
692Top K Frequent WordsHashPython
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846Hand of StraightsHashPython
347Top K Frequent ElementsHashPython
205Isomorphic StringsHashPython
268Missing NumberHashPython
242Valid AnagramHashPython
763Partition LabelsHashIntervalPython
383Ransom NoteHashPython
387 First Unique Character in a String HashPython
2186Minimum Number of Steps to Make Two Strings Anagram IIHashPython
282nd Weekly Contest
[Lint] 793Intersection of ArraysHashPython
146LRU CacheHashPython
387First Unique Character in a StringHashPython
[Lint] 920Meeting RoomsHashIntervalPython
#重要題型
[Lint] 920Meeting RoomsHashIntervalPython
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[Lint] 919Meeting Rooms IIHashIntervalPython
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[Lint] 919Meeting Rooms IIHashIntervalPython
#重要題型
349 Intersection of Two ArraysHashPython
217Contains DuplicateHashArrayPython
C++ 內含 C++ set, unordered_set 用法整理
137Single Number IIHashPython
350Intersection of Two Arrays IIHashPython
2248Intersection of Multiple ArraysHashPython
290th Weekly Contest
49Group AnagramsHash (Anagrams)Python
217Contains DuplicateHash (Duplicate)Python
128Longest Consecutive SequenceHash (LCS)Python
136Single NumberHash, Bit ManipulationPython
⭐ Heap 相關題型 ⭐
264Ugly Number IIHeapPython
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739Daily TemperaturesStackPython
496Next Greater Element IStackPython
503Next Greater Element IIStackPython
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283Move ZeroesTwo pointersPython
876Middle of the Linked ListTwo pointersPython
2260Minimum Consecutive Cards to Pick UpTwo pointers (快慢)Python
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21Merge Two Sorted ListsTwo pointers (Merge)Linked ListPython
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88Merge Sorted Array Two pointers (Merge)Python C++ 內含 python while-else 用法說明
1Two SumTwo pointers (NSum)ArrayPython C++ 內有 Python list comprehesion / dict comprehesion 整理 / C++ map find 方法補充 (dict find)
153Sum Two pointers (NSum)ArrayPython
163Sum ClosestTwo pointers (NSum)Python
167Two Sum II – Input Array Is SortedTwo pointers (NSum)Python
[Lint] 382Triangle CountTwo pointers (NSum)Python
[Lint] 533Two Sum – Closest to TargetTwo pointers (NSum)Python
[Lint] 587Two Sum – Unique PairsTwo pointers (NSum)Python
5Longest Palindromic Substring Two pointers (Palindrome)Python C++ 內含 C++ string.substr() 用法筆記
9Palindrome NumberTwo pointers (Palindrome)Python
125Valid PalindromeTwo pointers (Palindrome)StringPython內含 python isalpha(), isalnum() 的整理
680Valid Palindrome IITwo pointers (Palindrome)Python
409Longest Palindrome
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647Palindromic SubstringsTwo pointers (Palindrome)Python
234Palindrome Linked ListTwo pointers (Palindrome)Linked ListPython
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75Sort ColorsTwo pointers (partition)Python
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[Lint] 5Kth Largest ElementTwo pointers (partition)Python
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[Lint] 31Partition ArrayTwo pointers (partition)Python
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[Lint] 143Sort Colors IITwo pointers (partition)Python
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[Lint] 461Kth Smallest Numbers in Unsorted ArrayTwo pointers (partition)Python
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76 Minimum Window SubstringTwo pointers (Sliding Window)Python
239Sliding Window MaximumTwo pointers (Sliding Window)Python
⭐ 其他題型 / 待分類 ⭐
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48Rotate ImageMatrixPython
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238Product of Array Except SelfPython
222Count Complete Tree NodesTreePython
674Longest Continuous Increasing SubsequencePython
435Non-overlapping IntervalsIntervalPython
88Merge Sorted ArrayPython
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56Merge IntervalsIntervalPython
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61Rotate ListLinked ListPython
2259Remove Digit From Number to Maximize ResultPython
291st Weekly Contest
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54Spiral MatrixPython
228Summary RangesPython
263Ugly NumberPython
203Remove Linked List ElementsLinked ListPython
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2185Counting Words With a Given PrefixStringPython
282nd Weekly Contest
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121Best Time to Buy and Sell StockPython
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566Reshape the MatrixMatrixPython
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2243Calculate Digit Sum of a StringPython
289th Weekly Contest
2244Minimum Rounds to Complete All TasksPython
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2249Count Lattice Points Inside a CirclePython
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Howard Weng
Howard Weng

我是 Howard Weng,很多人叫我嗡嗡。這個網站放了我的各種筆記。希望這些筆記也能順便幫助到有需要的人們!如果文章有幫助到你的話,歡迎幫我點讚哦!
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