Counting LeetCode Problems: All 126,
With Python Solutions

Every problem in this library that LeetCode tags Counting. 112 of them carry a complete Python solution, a worked example and the time and space complexity of the approach.

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Easy: 47
Medium: 65
Hard: 14

1 pattern

How Counting problems are solved

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

Hash MapTrade memory for time: remember what you have seen so the second pass never happens.

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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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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

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

Counting problems by difficulty

Problems with a complete Python solution are listed first, then by ascending problem number.

Easy (47)

Easy problems
No.	Problem	Topics
0169	Majority Element	Array, Hash Table, Divide and Conquer +2
0383	Ransom Note	Hash Table, String, Counting
0387	First Unique Character in a String	Queue, Hash Table, String +1
0594	Longest Harmonious Subsequence	Array, Hash Table, Counting +2
0819	Most Common Word	Array, Hash Table, String +1
0884	Uncommon Words from Two Sentences	Hash Table, String, Counting
0914	X of a Kind in a Deck of Cards	Array, Hash Table, Math +2
1128	Number of Equivalent Domino Pairs	Array, Hash Table, Counting
1160	Find Words That Can Be Formed by Characters	Array, Hash Table, String +1
1189	Maximum Number of Balloons	Hash Table, String, Counting
1221	Split a String in Balanced Strings	Greedy, String, Counting
1356	Sort Integers by The Number of 1 Bits	Bit Manipulation, Array, Counting +1
1370	Increasing Decreasing String	Hash Table, String, Counting
1394	Find Lucky Integer in an Array	Array, Hash Table, Counting
1512	Number of Good Pairs	Array, Hash Table, Math +1
1603	Design Parking System	Design, Counting, Simulation
1684	Count the Number of Consistent Strings	Bit Manipulation, Array, Hash Table +2
1704	Determine if String Halves Are Alike	String, Counting
1742	Maximum Number of Balls in a Box	Hash Table, Math, Counting
1748	Sum of Unique Elements	Array, Hash Table, Counting
1790	Check if One String Swap Can Make Strings Equal	Hash Table, String, Counting
1854	Maximum Population Year	Array, Counting, Prefix Sum
1876	Substrings of Size Three with Distinct Characters	Hash Table, String, Counting +1
1897	Redistribute Characters to Make All Strings Equal	Hash Table, String, Counting
1941	Check if All Characters Have Equal Number of Occurrences	Hash Table, String, Counting
2006	Count Number of Pairs With Absolute Difference K	Array, Hash Table, Counting
2053	Kth Distinct String in an Array	Array, Hash Table, String +1
2068	Check Whether Two Strings are Almost Equivalent	Hash Table, String, Counting
2085	Count Common Words With One Occurrence	Array, Hash Table, String +1
2148	Count Elements With Strictly Smaller and Greater Elements	Array, Counting, Sorting
2190	Most Frequent Number Following Key In an Array	Array, Hash Table, Counting
2206	Divide Array Into Equal Pairs	Bit Manipulation, Array, Hash Table +1
2248	Intersection of Multiple Arrays	Array, Hash Table, Counting +1
2283	Check if Number Has Equal Digit Count and Digit Value	Hash Table, String, Counting
2287	Rearrange Characters to Make Target String	Hash Table, String, Counting
2341	Maximum Number of Pairs in Array	Array, Hash Table, Counting
2347	Best Poker Hand	Array, Hash Table, Counting
2351	First Letter to Appear Twice	Bit Manipulation, Hash Table, String +1
2404	Most Frequent Even Element	Array, Hash Table, Counting
2423	Remove Letter To Equalize Frequency	Hash Table, String, Counting
2506	Count Pairs Of Similar Strings	Bit Manipulation, Array, Hash Table +2
2529	Maximum Count of Positive Integer and Negative Integer	Array, Binary Search, Counting
2586	Count the Number of Vowel Strings in Range	Array, String, Counting
2748	Number of Beautiful Pairs	Array, Hash Table, Math +2
2833	Furthest Point From Origin	String, Counting
2960	Count Tested Devices After Test Operations	Array, Counting, Simulation
1213	Intersection of Three Sorted ArraysPremium	Array, Hash Table, Binary Search +1

Medium (65)

Medium problems
No.	Problem	Topics
0229	Majority Element II	Array, Hash Table, Counting +1
0299	Bulls and Cows	Hash Table, String, Counting
0347	Top K Frequent Elements	Array, Hash Table, Divide and Conquer +5
0451	Sort Characters By Frequency	Hash Table, String, Bucket Sort +3
0621	Task Scheduler	Greedy, Array, Hash Table +3
0692	Top K Frequent Words	Trie, Array, Hash Table +5
0767	Reorganize String	Greedy, Hash Table, String +3
0811	Subdomain Visit Count	Array, Hash Table, String +1
0869	Reordered Power of 2	Hash Table, Math, Counting +2
0900	RLE Iterator	Design, Array, Counting +1
0923	3Sum With Multiplicity	Array, Hash Table, Two Pointers +2
0945	Minimum Increment to Make Array Unique	Greedy, Array, Counting +1
1010	Pairs of Songs With Total Durations Divisible by 60	Array, Hash Table, Counting
1054	Distant Barcodes	Greedy, Array, Hash Table +3
1079	Letter Tile Possibilities	Hash Table, String, Backtracking +1
1090	Largest Values From Labels	Greedy, Array, Hash Table +2
1267	Count Servers that Communicate	Depth-First Search, Breadth-First Search, Union Find +3
1347	Minimum Number of Steps to Make Two Strings Anagram	Hash Table, String, Counting
1366	Rank Teams by Votes	Array, Hash Table, String +2
1400	Construct K Palindrome Strings	Greedy, Hash Table, String +1
1419	Minimum Number of Frogs Croaking	String, Counting
1481	Least Number of Unique Integers after K Removals	Greedy, Array, Hash Table +2
1497	Check If Array Pairs Are Divisible by k	Array, Hash Table, Counting
1519	Number of Nodes in the Sub-Tree With the Same Label	Tree, Depth-First Search, Breadth-First Search +2
1657	Determine if Two Strings Are Close	Hash Table, String, Counting +1
1726	Tuple with Same Product	Array, Hash Table, Counting
1737	Change Minimum Characters to Satisfy One of Three Conditions	Hash Table, String, Counting +1
1775	Equal Sum Arrays With Minimum Number of Operations	Greedy, Array, Hash Table +1
1781	Sum of Beauty of All Substrings	Hash Table, String, Counting
1814	Count Nice Pairs in an Array	Array, Hash Table, Math +1
2001	Number of Pairs of Interchangeable Rectangles	Array, Hash Table, Math +2
2013	Detect Squares	Design, Array, Hash Table +1
2023	Number of Pairs of Strings With Concatenation Equal to Target	Array, Hash Table, String +1
2029	Stone Game IX	Greedy, Array, Math +2
2048	Next Greater Numerically Balanced Number	Hash Table, Math, Backtracking +2
2131	Longest Palindrome by Concatenating Two Letter Words	Greedy, Array, Hash Table +2
2150	Find All Lonely Numbers in the Array	Array, Hash Table, Counting
2170	Minimum Operations to Make the Array Alternating	Greedy, Array, Hash Table +1
2182	Construct String With Repeat Limit	Greedy, Hash Table, String +2
2186	Minimum Number of Steps to Make Two Strings Anagram II	Hash Table, String, Counting
2225	Find Players With Zero or One Losses	Array, Hash Table, Counting +1
2244	Minimum Rounds to Complete All Tasks	Greedy, Array, Hash Table +1
2275	Largest Combination With Bitwise AND Greater Than Zero	Bit Manipulation, Array, Hash Table +1
2284	Sender With Largest Word Count	Array, Hash Table, String +1
2364	Count Number of Bad Pairs	Array, Hash Table, Math +1
2384	Largest Palindromic Number	Greedy, Hash Table, String +1
2442	Count Number of Distinct Integers After Reverse Operations	Array, Hash Table, Math +1
2453	Destroy Sequential Targets	Array, Hash Table, Counting
2526	Find Consecutive Integers from a Data Stream	Design, Queue, Hash Table +2
2531	Make Number of Distinct Characters Equal	Hash Table, String, Counting
2856	Minimum Array Length After Pair Removals	Greedy, Array, Hash Table +3
2870	Minimum Number of Operations to Make Array Empty	Greedy, Array, Hash Table +1
2981	Find Longest Special Substring That Occurs Thrice I	Hash Table, String, Binary Search +2
2982	Find Longest Special Substring That Occurs Thrice II	Hash Table, String, Binary Search +2
1198	Find Smallest Common Element in All RowsPremium	Array, Hash Table, Binary Search +2
1612	Check If Two Expression Trees are EquivalentPremium	Tree, Depth-First Search, Hash Table +2
1940	Longest Common Subsequence Between Sorted ArraysPremium	Array, Hash Table, Counting
2067	Number of Equal Count SubstringsPremium	Hash Table, String, Counting +1
2083	Substrings That Begin and End With the Same LetterPremium	Hash Table, Math, String +2
2168	Unique Substrings With Equal Digit FrequencyPremium	Hash Table, String, Counting +2
2268	Minimum Number of KeypressesPremium	Greedy, Hash Table, String +2
2539	Count the Number of Good SubsequencesPremium	Hash Table, Math, String +2
2782	Number of Unique CategoriesPremium	Union Find, Counting, Interactive
2950	Number of Divisible SubstringsPremium	Hash Table, String, Counting +1
2955	Number of Same-End SubstringsPremium	Array, Hash Table, String +2

Hard (14)

Hard problems
No.	Problem	Topics
0992	Subarrays with K Different Integers	Array, Hash Table, Counting +1
1255	Maximum Score Words Formed by Letters	Bit Manipulation, Array, Hash Table +5
1782	Count Pairs Of Nodes	Graph, Array, Hash Table +4
1819	Number of Different Subsequences GCDs	Array, Math, Counting +1
1857	Largest Color Value in a Directed Graph	Graph, Topological Sort, Memoization +3
1994	The Number of Good Subsets	Bit Manipulation, Array, Hash Table +5
2014	Longest Subsequence Repeated k Times	Greedy, String, Backtracking +2
2025	Maximum Number of Ways to Partition an Array	Array, Hash Table, Counting +2
2416	Sum of Prefix Scores of Strings	Trie, Array, String +1
2499	Minimum Total Cost to Make Arrays Unequal	Greedy, Array, Hash Table +1
2514	Count Anagrams	Hash Table, Math, String +2
2547	Minimum Cost to Split an Array	Array, Hash Table, Dynamic Programming +1
0358	Rearrange String k Distance ApartPremium	Greedy, Hash Table, String +3
1121	Divide Array Into Increasing SequencesPremium	Array, Counting

How Counting problems are solved

When the Counting problem arrives live.

Approach

Approach

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Counting problems by difficulty

Easy (47)47

Medium (65)65

Hard (14)14

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Easy (47)

Medium (65)

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