Easy LeetCode Problems: All 760,
With Python Solutions

Easy problems test whether you can turn a stated requirement into working code without fumbling the mechanics. The technique is usually named in the problem; the work is the edge cases.

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The patterns behind easy problems

The six patterns with the most Easy-rated problems in this library. Working through a pattern beats working down a difficulty list, because the technique transfers and a difficulty band does not.

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python

Please analyze this context and help me answer the question or solve the problem.

Technical interview · 45 minsolution.pyShared editor · Python 3Longest substring without repeatsMediumGiven a string s, return the length of the longest contiguous substring in which every character is unique.s = "abcabcbb"
→ 31def length_of_longest_substring(s: str) -> int:2 last_seen = {}3 left = best = 04 for right, ch in enumerate(s):5 if ch in last_seen and last_seen[ch] >= left:

python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

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GPT-6.1 Terra

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python

Please analyze this context and help me answer the question or solve the problem.

python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

Type your message...

GPT-6.1 TerraClear chat

QuitEscHide+QCapture+PCrop+OSubmit+EnterPress to get answerLeft/Right+Up/Down+

90%Subscription

python

Please analyze this context and help me answer the question or solve the problem.

python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

Type your message...

GPT-6.1 TerraClear chat

Primary display

QuitEscHide+QCapture+PCrop+OSubmit+EnterPress to get answerLeft/Right+Up/Down+

Page 1 of 4

Easy problems

Showing the first 200 of 760 problems. Problems with a complete Python solution are listed first, then by ascending problem number.

Easy problems
No.	Problem	Topics
0001	Two Sum	Array, Hash Table
0009	Palindrome Number	Math
0013	Roman to Integer	Hash Table, Math, String
0014	Longest Common Prefix	Trie, Array, String
0020	Valid Parentheses	Stack, String
0021	Merge Two Sorted Lists	Recursion, Linked List
0026	Remove Duplicates from Sorted Array	Array, Two Pointers
0027	Remove Element	Array, Two Pointers
0028	Find the Index of the First Occurrence in a String	Two Pointers, String, String Matching
0035	Search Insert Position	Array, Binary Search
0058	Length of Last Word	String
0066	Plus One	Array, Math
0067	Add Binary	Bit Manipulation, Math, String +1
0069	Sqrt(x)	Math, Binary Search
0070	Climbing Stairs	Memoization, Math, Dynamic Programming
0083	Remove Duplicates from Sorted List	Linked List
0088	Merge Sorted Array	Array, Two Pointers, Sorting
0094	Binary Tree Inorder Traversal	Stack, Tree, Depth-First Search +1
0100	Same Tree	Tree, Depth-First Search, Breadth-First Search +1
0101	Symmetric Tree	Tree, Depth-First Search, Breadth-First Search +1
0104	Maximum Depth of Binary Tree	Tree, Depth-First Search, Breadth-First Search +1
0108	Convert Sorted Array to Binary Search Tree	Tree, Binary Search Tree, Array +2
0110	Balanced Binary Tree	Tree, Depth-First Search, Binary Tree
0111	Minimum Depth of Binary Tree	Tree, Depth-First Search, Breadth-First Search +1
0112	Path Sum	Tree, Depth-First Search, Breadth-First Search +1
0118	Pascal's Triangle	Array, Dynamic Programming
0119	Pascal's Triangle II	Array, Dynamic Programming
0121	Best Time to Buy and Sell Stock	Array, Dynamic Programming
0125	Valid Palindrome	Two Pointers, String
0136	Single Number	Bit Manipulation, Array
0141	Linked List Cycle	Hash Table, Linked List, Two Pointers
0144	Binary Tree Preorder Traversal	Stack, Tree, Depth-First Search +1
0145	Binary Tree Postorder Traversal	Stack, Tree, Depth-First Search +1
0160	Intersection of Two Linked Lists	Hash Table, Linked List, Two Pointers
0168	Excel Sheet Column Title	Math, String
0169	Majority Element	Array, Hash Table, Divide and Conquer +2
0171	Excel Sheet Column Number	Math, String
0175	Combine Two Tables	Database
0181	Employees Earning More Than Their Managers	Database
0182	Duplicate Emails	Database
0183	Customers Who Never Order	Database
0190	Reverse Bits	Bit Manipulation, Divide and Conquer
0191	Number of 1 Bits	Bit Manipulation, Divide and Conquer
0193	Valid Phone Numbers	Shell
0195	Tenth Line	Shell
0196	Delete Duplicate Emails	Database
0197	Rising Temperature	Database
0202	Happy Number	Hash Table, Math, Two Pointers
0203	Remove Linked List Elements	Recursion, Linked List
0205	Isomorphic Strings	Hash Table, String
0206	Reverse Linked List	Recursion, Linked List
0217	Contains Duplicate	Array, Hash Table, Sorting
0219	Contains Duplicate II	Array, Hash Table, Sliding Window
0222	Count Complete Tree Nodes	Bit Manipulation, Tree, Binary Search +1
0225	Implement Stack using Queues	Stack, Design, Queue
0226	Invert Binary Tree	Tree, Depth-First Search, Breadth-First Search +1
0228	Summary Ranges	Array
0231	Power of Two	Bit Manipulation, Recursion, Math
0232	Implement Queue using Stacks	Stack, Design, Queue
0234	Palindrome Linked List	Stack, Recursion, Linked List +1
0242	Valid Anagram	Hash Table, String, Sorting
0257	Binary Tree Paths	Tree, Depth-First Search, String +2
0258	Add Digits	Math, Number Theory, Simulation
0263	Ugly Number	Math
0268	Missing Number	Bit Manipulation, Array, Hash Table +3
0278	First Bad Version	Binary Search, Interactive
0283	Move Zeroes	Array, Two Pointers
0290	Word Pattern	Hash Table, String
0292	Nim Game	Brainteaser, Math, Game Theory
0303	Range Sum Query - Immutable	Design, Array, Prefix Sum
0326	Power of Three	Recursion, Math
0338	Counting Bits	Bit Manipulation, Dynamic Programming
0342	Power of Four	Bit Manipulation, Recursion, Math
0344	Reverse String	Two Pointers, String
0345	Reverse Vowels of a String	Two Pointers, String
0349	Intersection of Two Arrays	Array, Hash Table, Two Pointers +2
0350	Intersection of Two Arrays II	Array, Hash Table, Two Pointers +2
0367	Valid Perfect Square	Math, Binary Search
0374	Guess Number Higher or Lower	Binary Search, Interactive
0383	Ransom Note	Hash Table, String, Counting
0387	First Unique Character in a String	Queue, Hash Table, String +1
0389	Find the Difference	Bit Manipulation, Hash Table, String +1
0392	Is Subsequence	Two Pointers, String, Dynamic Programming
0401	Binary Watch	Bit Manipulation, Backtracking
0404	Sum of Left Leaves	Tree, Depth-First Search, Breadth-First Search +1
0405	Convert a Number to Hexadecimal	Bit Manipulation, Math, String
0409	Longest Palindrome	Greedy, Hash Table, String
0412	Fizz Buzz	Math, String, Simulation
0414	Third Maximum Number	Array, Sorting
0415	Add Strings	Math, String, Simulation
0434	Number of Segments in a String	String
0441	Arranging Coins	Math, Binary Search
0448	Find All Numbers Disappeared in an Array	Array, Hash Table
0455	Assign Cookies	Greedy, Array, Two Pointers +1
0459	Repeated Substring Pattern	String, String Matching
0461	Hamming Distance	Bit Manipulation
0463	Island Perimeter	Depth-First Search, Breadth-First Search, Array +1
0476	Number Complement	Bit Manipulation
0482	License Key Formatting	String
0485	Max Consecutive Ones	Array
0492	Construct the Rectangle	Math
0495	Teemo Attacking	Array, Simulation
0496	Next Greater Element I	Stack, Array, Hash Table +1
0500	Keyboard Row	Array, Hash Table, String
0501	Find Mode in Binary Search Tree	Tree, Depth-First Search, Binary Search Tree +1
0504	Base 7	Math, String
0506	Relative Ranks	Array, Sorting, Heap (Priority Queue)
0507	Perfect Number	Math
0509	Fibonacci Number	Recursion, Memoization, Math +1
0511	Game Play Analysis I	Database
0520	Detect Capital	String
0521	Longest Uncommon Subsequence I	String
0530	Minimum Absolute Difference in BST	Tree, Depth-First Search, Breadth-First Search +2
0541	Reverse String II	Two Pointers, String
0543	Diameter of Binary Tree	Tree, Depth-First Search, Binary Tree
0551	Student Attendance Record I	String
0557	Reverse Words in a String III	Two Pointers, String
0559	Maximum Depth of N-ary Tree	Tree, Depth-First Search, Breadth-First Search
0561	Array Partition	Greedy, Array, Counting Sort +1
0563	Binary Tree Tilt	Tree, Depth-First Search, Binary Tree
0566	Reshape the Matrix	Array, Matrix, Simulation
0572	Subtree of Another Tree	Tree, Depth-First Search, Binary Tree +2
0575	Distribute Candies	Array, Hash Table
0577	Employee Bonus	Database
0584	Find Customer Referee	Database
0586	Customer Placing the Largest Number of Orders	Database
0589	N-ary Tree Preorder Traversal	Stack, Tree, Depth-First Search
0590	N-ary Tree Postorder Traversal	Stack, Tree, Depth-First Search
0594	Longest Harmonious Subsequence	Array, Hash Table, Counting +2
0595	Big Countries	Database
0596	Classes With at Least 5 Students	Database
0598	Range Addition II	Array, Math
0599	Minimum Index Sum of Two Lists	Array, Hash Table, String
0605	Can Place Flowers	Greedy, Array
0607	Sales Person	Database
0610	Triangle Judgement	Database
0617	Merge Two Binary Trees	Tree, Depth-First Search, Breadth-First Search +1
0619	Biggest Single Number	Database
0620	Not Boring Movies	Database
0627	Swap Sex of Employees	Database
0628	Maximum Product of Three Numbers	Array, Math, Sorting
0637	Average of Levels in Binary Tree	Tree, Depth-First Search, Breadth-First Search +1
0643	Maximum Average Subarray I	Array, Sliding Window
0645	Set Mismatch	Bit Manipulation, Array, Hash Table +1
0653	Two Sum IV - Input is a BST	Tree, Depth-First Search, Breadth-First Search +4
0657	Robot Return to Origin	String, Simulation
0661	Image Smoother	Array, Matrix
0671	Second Minimum Node In a Binary Tree	Tree, Depth-First Search, Binary Tree
0674	Longest Continuous Increasing Subsequence	Array
0680	Valid Palindrome II	Greedy, Two Pointers, String
0682	Baseball Game	Stack, Array, Simulation
0693	Binary Number with Alternating Bits	Bit Manipulation
0696	Count Binary Substrings	Two Pointers, String
0697	Degree of an Array	Array, Hash Table
0700	Search in a Binary Search Tree	Tree, Binary Search Tree, Binary Tree
0703	Kth Largest Element in a Stream	Tree, Design, Binary Search Tree +3
0704	Binary Search	Array, Binary Search
0705	Design HashSet	Design, Array, Hash Table +2
0706	Design HashMap	Design, Array, Hash Table +2
0709	To Lower Case	String
0717	1-bit and 2-bit Characters	Array
0724	Find Pivot Index	Array, Prefix Sum
0728	Self Dividing Numbers	Math
0733	Flood Fill	Depth-First Search, Breadth-First Search, Array +1
0744	Find Smallest Letter Greater Than Target	Array, Binary Search
0746	Min Cost Climbing Stairs	Array, Dynamic Programming
0747	Largest Number At Least Twice of Others	Array, Sorting
0748	Shortest Completing Word	Array, Hash Table, String
0762	Prime Number of Set Bits in Binary Representation	Bit Manipulation, Math
0766	Toeplitz Matrix	Array, Matrix
0771	Jewels and Stones	Hash Table, String
0783	Minimum Distance Between BST Nodes	Tree, Depth-First Search, Breadth-First Search +2
0796	Rotate String	String, String Matching
0804	Unique Morse Code Words	Array, Hash Table, String
0806	Number of Lines To Write String	Array, String
0812	Largest Triangle Area	Geometry, Array, Math
0819	Most Common Word	Array, Hash Table, String +1
0821	Shortest Distance to a Character	Array, Two Pointers, String
0824	Goat Latin	String
0830	Positions of Large Groups	String
0832	Flipping an Image	Bit Manipulation, Array, Two Pointers +2
0836	Rectangle Overlap	Geometry, Math
0844	Backspace String Compare	Stack, Two Pointers, String +1
0859	Buddy Strings	Hash Table, String
0860	Lemonade Change	Greedy, Array
0867	Transpose Matrix	Array, Matrix, Simulation
0868	Binary Gap	Bit Manipulation
0872	Leaf-Similar Trees	Tree, Depth-First Search, Binary Tree
0876	Middle of the Linked List	Linked List, Two Pointers
0883	Projection Area of 3D Shapes	Geometry, Array, Math +1
0884	Uncommon Words from Two Sentences	Hash Table, String, Counting
0888	Fair Candy Swap	Array, Hash Table, Binary Search +1
0892	Surface Area of 3D Shapes	Geometry, Array, Math +1
0896	Monotonic Array	Array
0897	Increasing Order Search Tree	Stack, Tree, Depth-First Search +2
0905	Sort Array By Parity	Array, Two Pointers, Sorting
0908	Smallest Range I	Array, Math
0914	X of a Kind in a Deck of Cards	Array, Hash Table, Math +2
0917	Reverse Only Letters	Two Pointers, String
0922	Sort Array By Parity II	Array, Two Pointers, Sorting

The patterns behind easy problems

An easy problem, live, with someone watching.

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Easy problems

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