Two Pointers LeetCode Problems: All 201,
With Python Solutions

Every problem in this library that LeetCode tags Two Pointers. 169 of them carry a complete Python solution, a worked example and the time and space complexity of the approach.

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Easy: 58
Medium: 118
Hard: 25

1 pattern

How Two Pointers problems are solved

A tag names the subject, not the method. These pattern hubs cover the techniques that actually solve Two Pointers problems — each one explains the approach, gives a Python template and states its complexity.

Two PointersUse the order already in the input to discard half the search space at every step.

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python

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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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python

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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...

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

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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

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

Two Pointers problems by difficulty

Showing the first 200 of 201 problems. Problems with a complete Python solution are listed first, then by ascending problem number.

Easy (58)

Easy problems
No.	Problem	Topics
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
0088	Merge Sorted Array	Array, Two Pointers, Sorting
0125	Valid Palindrome	Two Pointers, String
0141	Linked List Cycle	Hash Table, Linked List, Two Pointers
0160	Intersection of Two Linked Lists	Hash Table, Linked List, Two Pointers
0202	Happy Number	Hash Table, Math, Two Pointers
0234	Palindrome Linked List	Stack, Recursion, Linked List +1
0283	Move Zeroes	Array, Two Pointers
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
0392	Is Subsequence	Two Pointers, String, Dynamic Programming
0455	Assign Cookies	Greedy, Array, Two Pointers +1
0541	Reverse String II	Two Pointers, String
0557	Reverse Words in a String III	Two Pointers, String
0653	Two Sum IV - Input is a BST	Tree, Depth-First Search, Breadth-First Search +4
0680	Valid Palindrome II	Greedy, Two Pointers, String
0696	Count Binary Substrings	Two Pointers, String
0821	Shortest Distance to a Character	Array, Two Pointers, String
0832	Flipping an Image	Bit Manipulation, Array, Two Pointers +2
0844	Backspace String Compare	Stack, Two Pointers, String +1
0876	Middle of the Linked List	Linked List, Two Pointers
0905	Sort Array By Parity	Array, Two Pointers, Sorting
0917	Reverse Only Letters	Two Pointers, String
0922	Sort Array By Parity II	Array, Two Pointers, Sorting
0925	Long Pressed Name	Two Pointers, String
0942	DI String Match	Greedy, Array, Two Pointers +1
0977	Squares of a Sorted Array	Array, Two Pointers, Sorting
1089	Duplicate Zeros	Array, Two Pointers
1332	Remove Palindromic Subsequences	Two Pointers, String
1346	Check If N and Its Double Exist	Array, Hash Table, Two Pointers +2
1385	Find the Distance Value Between Two Arrays	Array, Two Pointers, Binary Search +1
1455	Check If a Word Occurs As a Prefix of Any Word in a Sentence	Two Pointers, String, String Matching
1768	Merge Strings Alternately	Two Pointers, String
1961	Check If String Is a Prefix of Array	Array, Two Pointers, String
2000	Reverse Prefix of Word	Stack, Two Pointers, String
2108	Find First Palindromic String in the Array	Array, Two Pointers, String
2200	Find All K-Distant Indices in an Array	Array, Two Pointers
2367	Number of Arithmetic Triplets	Array, Hash Table, Two Pointers +1
2441	Largest Positive Integer That Exists With Its Negative	Array, Hash Table, Two Pointers +1
2460	Apply Operations to an Array	Array, Two Pointers, Simulation
2465	Number of Distinct Averages	Array, Hash Table, Two Pointers +1
2511	Maximum Enemy Forts That Can Be Captured	Array, Two Pointers
2540	Minimum Common Value	Array, Hash Table, Two Pointers +1
2562	Find the Array Concatenation Value	Array, Two Pointers, Simulation
2570	Merge Two 2D Arrays by Summing Values	Array, Hash Table, Two Pointers
2697	Lexicographically Smallest Palindrome	Greedy, Two Pointers, String
2824	Count Pairs Whose Sum is Less than Target	Array, Two Pointers, Binary Search +1
2903	Find Indices With Index and Value Difference I	Array, Two Pointers
2970	Count the Number of Incremovable Subarrays I	Array, Two Pointers, Binary Search +1
0170	Two Sum III - Data structure designPremium	Design, Array, Hash Table +2
0246	Strobogrammatic NumberPremium	Hash Table, Two Pointers, String
0408	Valid Word AbbreviationPremium	Two Pointers, String
1099	Two Sum Less Than KPremium	Array, Two Pointers, Binary Search +1
1826	Faulty SensorPremium	Array, Two Pointers

Medium (118)

Medium problems
No.	Problem	Topics
0005	Longest Palindromic Substring	Two Pointers, String, Dynamic Programming
0011	Container With Most Water	Greedy, Array, Two Pointers
0015	3Sum	Array, Two Pointers, Sorting
0016	3Sum Closest	Array, Two Pointers, Sorting
0018	4Sum	Array, Two Pointers, Sorting
0019	Remove Nth Node From End of List	Linked List, Two Pointers
0031	Next Permutation	Array, Two Pointers
0061	Rotate List	Linked List, Two Pointers
0075	Sort Colors	Array, Two Pointers, Sorting
0080	Remove Duplicates from Sorted Array II	Array, Two Pointers
0082	Remove Duplicates from Sorted List II	Linked List, Two Pointers
0086	Partition List	Linked List, Two Pointers
0142	Linked List Cycle II	Hash Table, Linked List, Two Pointers
0143	Reorder List	Stack, Recursion, Linked List +1
0148	Sort List	Linked List, Two Pointers, Divide and Conquer +2
0151	Reverse Words in a String	Two Pointers, String
0165	Compare Version Numbers	Two Pointers, String
0167	Two Sum II - Input Array Is Sorted	Array, Two Pointers, Binary Search
0189	Rotate Array	Array, Math, Two Pointers
0287	Find the Duplicate Number	Bit Manipulation, Array, Two Pointers +1
0443	String Compression	Two Pointers, String
0457	Circular Array Loop	Array, Hash Table, Two Pointers
0475	Heaters	Array, Two Pointers, Binary Search +1
0481	Magical String	Two Pointers, String
0522	Longest Uncommon Subsequence II	Array, Hash Table, Two Pointers +2
0524	Longest Word in Dictionary through Deleting	Array, Two Pointers, String +1
0532	K-diff Pairs in an Array	Array, Hash Table, Two Pointers +2
0556	Next Greater Element III	Math, Two Pointers, String
0567	Permutation in String	Hash Table, Two Pointers, String +1
0581	Shortest Unsorted Continuous Subarray	Stack, Greedy, Array +3
0611	Valid Triangle Number	Greedy, Array, Two Pointers +2
0633	Sum of Square Numbers	Math, Two Pointers, Binary Search
0647	Palindromic Substrings	Two Pointers, String, Dynamic Programming
0658	Find K Closest Elements	Array, Two Pointers, Binary Search +3
0763	Partition Labels	Greedy, Hash Table, Two Pointers +1
0777	Swap Adjacent in LR String	Two Pointers, String
0786	K-th Smallest Prime Fraction	Array, Two Pointers, Binary Search +2
0795	Number of Subarrays with Bounded Maximum	Array, Two Pointers
0809	Expressive Words	Array, Two Pointers, String
0825	Friends Of Appropriate Ages	Array, Two Pointers, Binary Search +1
0826	Most Profit Assigning Work	Greedy, Array, Two Pointers +2
0838	Push Dominoes	Two Pointers, String, Dynamic Programming
0845	Longest Mountain in Array	Array, Two Pointers, Dynamic Programming +1
0870	Advantage Shuffle	Greedy, Array, Two Pointers +1
0881	Boats to Save People	Greedy, Array, Two Pointers +1
0923	3Sum With Multiplicity	Array, Hash Table, Two Pointers +2
0948	Bag of Tokens	Greedy, Array, Two Pointers +1
0962	Maximum Width Ramp	Stack, Array, Two Pointers +1
0969	Pancake Sorting	Greedy, Array, Two Pointers +1
0986	Interval List Intersections	Array, Two Pointers, Line Sweep
1023	Camelcase Matching	Trie, Array, Two Pointers +2
1048	Longest String Chain	Array, Hash Table, Two Pointers +3
1237	Find Positive Integer Solution for a Given Equation	Math, Two Pointers, Binary Search +1
1471	The k Strongest Values in an Array	Array, Two Pointers, Sorting
1498	Number of Subsequences That Satisfy the Given Sum Condition	Array, Two Pointers, Binary Search +1
1508	Range Sum of Sorted Subarray Sums	Array, Two Pointers, Binary Search +2
1574	Shortest Subarray to be Removed to Make Array Sorted	Stack, Array, Two Pointers +2
1577	Number of Ways Where Square of Number Is Equal to Product of Two Numbers	Array, Hash Table, Math +1
1616	Split Two Strings to Make Palindrome	Two Pointers, String
1679	Max Number of K-Sum Pairs	Array, Hash Table, Two Pointers +1
1712	Ways to Split Array Into Three Subarrays	Array, Two Pointers, Binary Search +1
1721	Swapping Nodes in a Linked List	Linked List, Two Pointers
1750	Minimum Length of String After Deleting Similar Ends	Two Pointers, String
1754	Largest Merge Of Two Strings	Greedy, Two Pointers, String
1764	Form Array by Concatenating Subarrays of Another Array	Greedy, Array, Two Pointers +1
1813	Sentence Similarity III	Array, Two Pointers, String
1850	Minimum Adjacent Swaps to Reach the Kth Smallest Number	Greedy, Two Pointers, String
1855	Maximum Distance Between a Pair of Values	Array, Two Pointers, Binary Search
1861	Rotating the Box	Array, Two Pointers, Matrix
1877	Minimize Maximum Pair Sum in Array	Greedy, Array, Two Pointers +1
1898	Maximum Number of Removable Characters	Array, Two Pointers, String +1
1963	Minimum Number of Swaps to Make the String Balanced	Stack, Greedy, Two Pointers +1
2095	Delete the Middle Node of a Linked List	Linked List, Two Pointers
2105	Watering Plants II	Array, Two Pointers, Simulation
2109	Adding Spaces to a String	Array, Two Pointers, String +1
2110	Number of Smooth Descent Periods of a Stock	Array, Math, Two Pointers +2
2130	Maximum Twin Sum of a Linked List	Stack, Linked List, Two Pointers
2149	Rearrange Array Elements by Sign	Array, Two Pointers, Simulation
2161	Partition Array According to Given Pivot	Array, Two Pointers, Simulation
2300	Successful Pairs of Spells and Potions	Array, Two Pointers, Binary Search +1
2332	The Latest Time to Catch a Bus	Array, Two Pointers, Binary Search +1
2337	Move Pieces to Obtain a String	Two Pointers, String
2396	Strictly Palindromic Number	Brainteaser, Math, Two Pointers
2406	Divide Intervals Into Minimum Number of Groups	Greedy, Array, Two Pointers +3
2410	Maximum Matching of Players With Trainers	Greedy, Array, Two Pointers +1
2462	Total Cost to Hire K Workers	Array, Two Pointers, Simulation +1
2486	Append Characters to String to Make Subsequence	Greedy, Two Pointers, String
2491	Divide Players Into Teams of Equal Skill	Array, Hash Table, Two Pointers +1
2563	Count the Number of Fair Pairs	Array, Two Pointers, Binary Search +1
2576	Find the Maximum Number of Marked Indices	Greedy, Array, Two Pointers +2
2592	Maximize Greatness of an Array	Greedy, Array, Two Pointers +1
2825	Make String a Subsequence Using Cyclic Increments	Two Pointers, String
2856	Minimum Array Length After Pair Removals	Greedy, Array, Hash Table +3
2905	Find Indices With Index and Value Difference II	Array, Two Pointers
2938	Separate Black and White Balls	Greedy, Two Pointers, String
0161	One Edit DistancePremium	Two Pointers, String
0186	Reverse Words in a String IIPremium	Two Pointers, String
0244	Shortest Word Distance IIPremium	Design, Array, Hash Table +2
0251	Flatten 2D VectorPremium	Design, Array, Two Pointers +1
0253	Meeting Rooms IIPremium	Greedy, Array, Two Pointers +3
0259	3Sum SmallerPremium	Array, Two Pointers, Binary Search +1
0277	Find the CelebrityPremium	Graph, Two Pointers, Interactive
0360	Sort Transformed ArrayPremium	Array, Math, Two Pointers +1
0723	Candy CrushPremium	Array, Two Pointers, Matrix +1
1055	Shortest Way to Form StringPremium	Greedy, Two Pointers, String +1
1214	Two Sum BSTsPremium	Stack, Tree, Depth-First Search +4
1229	Meeting SchedulerPremium	Array, Two Pointers, Sorting
1265	Print Immutable Linked List in ReversePremium	Stack, Recursion, Linked List +1
1570	Dot Product of Two Sparse VectorsPremium	Design, Array, Hash Table +1
1634	Add Two Polynomials Represented as Linked ListsPremium	Linked List, Math, Two Pointers
1650	Lowest Common Ancestor of a Binary Tree IIIPremium	Tree, Hash Table, Two Pointers +1
1868	Product of Two Run-Length Encoded ArraysPremium	Array, Two Pointers
1885	Count Pairs in Two ArraysPremium	Array, Two Pointers, Binary Search +1
2046	Sort Linked List Already Sorted Using Absolute ValuesPremium	Linked List, Two Pointers, Sorting
2330	Valid Palindrome IVPremium	Two Pointers, String
2422	Merge Operations to Turn Array Into a PalindromePremium	Greedy, Array, Two Pointers
2674	Split a Circular Linked ListPremium	Linked List, Two Pointers
2838	Maximum Coins Heroes Can CollectPremium	Array, Two Pointers, Binary Search +2

Hard (24)

Hard problems
No.	Problem	Topics
0042	Trapping Rain Water	Stack, Array, Two Pointers +2
0295	Find Median from Data Stream	Design, Two Pointers, Data Stream +2
0321	Create Maximum Number	Stack, Greedy, Array +2
0719	Find K-th Smallest Pair Distance	Array, Two Pointers, Binary Search +1
1147	Longest Chunked Palindrome Decomposition	Greedy, Two Pointers, String +3
1163	Last Substring in Lexicographical Order	Two Pointers, String
1537	Get the Maximum Score	Greedy, Array, Two Pointers +1
1697	Checking Existence of Edge Length Limited Paths	Union Find, Graph, Array +2
1755	Closest Subsequence Sum	Bit Manipulation, Array, Two Pointers +3
1782	Count Pairs Of Nodes	Graph, Array, Hash Table +4
1793	Maximum Score of a Good Subarray	Stack, Array, Two Pointers +2
2035	Partition Array Into Two Arrays to Minimize Sum Difference	Bit Manipulation, Array, Two Pointers +4
2071	Maximum Number of Tasks You Can Assign	Greedy, Queue, Array +4
2122	Recover the Original Array	Array, Hash Table, Two Pointers +2
2193	Minimum Number of Moves to Make Palindrome	Greedy, Binary Indexed Tree, Two Pointers +1
2234	Maximum Total Beauty of the Gardens	Greedy, Array, Two Pointers +4
2472	Maximum Number of Non-overlapping Palindrome Substrings	Greedy, Two Pointers, String +1
2503	Maximum Number of Points From Grid Queries	Breadth-First Search, Union Find, Array +4
2565	Subsequence With the Minimum Score	Two Pointers, String, Binary Search
2911	Minimum Changes to Make K Semi-palindromes	Two Pointers, String, Dynamic Programming
2972	Count the Number of Incremovable Subarrays II	Array, Two Pointers, Binary Search
0272	Closest Binary Search Tree Value IIPremium	Stack, Tree, Depth-First Search +4
1842	Next Palindrome Using Same DigitsPremium	Two Pointers, String
2604	Minimum Time to Eat All GrainsPremium	Array, Two Pointers, Binary Search +1

How Two Pointers problems are solved

When the Two Pointers problem arrives live.

Approach

Approach

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Two Pointers problems by difficulty

Easy (58)58

Medium (118)118

Hard (24)24

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Easy (58)

Medium (118)

Hard (24)