Counting LeetCode Problems: All 126, With Python Solutions
Every problem in this library that LeetCode tags Counting — 126 in total, 112 of them with a complete Python solution, a worked example and the time and space complexity of the approach.
- 126 problems
- 47 Easy
- 65 Medium
- 14 Hard
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 Map — Trade memory for time: remember what you have seen so the second pass never happens.
Counting problems by difficulty
Problems with a complete Python solution are listed first, then by ascending problem number.
Easy (47)
| # | Problem | Difficulty | Topics |
|---|---|---|---|
| 169 | Majority Element | Easy | Array, Hash Table, Divide and Conquer +2 |
| 383 | Ransom Note | Easy | Hash Table, String, Counting |
| 387 | First Unique Character in a String | Easy | Queue, Hash Table, String +1 |
| 594 | Longest Harmonious Subsequence | Easy | Array, Hash Table, Counting +2 |
| 819 | Most Common Word | Easy | Array, Hash Table, String +1 |
| 884 | Uncommon Words from Two Sentences | Easy | Hash Table, String, Counting |
| 914 | X of a Kind in a Deck of Cards | Easy | Array, Hash Table, Math +2 |
| 1128 | Number of Equivalent Domino Pairs | Easy | Array, Hash Table, Counting |
| 1160 | Find Words That Can Be Formed by Characters | Easy | Array, Hash Table, String +1 |
| 1189 | Maximum Number of Balloons | Easy | Hash Table, String, Counting |
| 1221 | Split a String in Balanced Strings | Easy | Greedy, String, Counting |
| 1356 | Sort Integers by The Number of 1 Bits | Easy | Bit Manipulation, Array, Counting +1 |
| 1370 | Increasing Decreasing String | Easy | Hash Table, String, Counting |
| 1394 | Find Lucky Integer in an Array | Easy | Array, Hash Table, Counting |
| 1512 | Number of Good Pairs | Easy | Array, Hash Table, Math +1 |
| 1603 | Design Parking System | Easy | Design, Counting, Simulation |
| 1684 | Count the Number of Consistent Strings | Easy | Bit Manipulation, Array, Hash Table +2 |
| 1704 | Determine if String Halves Are Alike | Easy | String, Counting |
| 1742 | Maximum Number of Balls in a Box | Easy | Hash Table, Math, Counting |
| 1748 | Sum of Unique Elements | Easy | Array, Hash Table, Counting |
| 1790 | Check if One String Swap Can Make Strings Equal | Easy | Hash Table, String, Counting |
| 1854 | Maximum Population Year | Easy | Array, Counting, Prefix Sum |
| 1876 | Substrings of Size Three with Distinct Characters | Easy | Hash Table, String, Counting +1 |
| 1897 | Redistribute Characters to Make All Strings Equal | Easy | Hash Table, String, Counting |
| 1941 | Check if All Characters Have Equal Number of Occurrences | Easy | Hash Table, String, Counting |
| 2006 | Count Number of Pairs With Absolute Difference K | Easy | Array, Hash Table, Counting |
| 2053 | Kth Distinct String in an Array | Easy | Array, Hash Table, String +1 |
| 2068 | Check Whether Two Strings are Almost Equivalent | Easy | Hash Table, String, Counting |
| 2085 | Count Common Words With One Occurrence | Easy | Array, Hash Table, String +1 |
| 2148 | Count Elements With Strictly Smaller and Greater Elements | Easy | Array, Counting, Sorting |
| 2190 | Most Frequent Number Following Key In an Array | Easy | Array, Hash Table, Counting |
| 2206 | Divide Array Into Equal Pairs | Easy | Bit Manipulation, Array, Hash Table +1 |
| 2248 | Intersection of Multiple Arrays | Easy | Array, Hash Table, Counting +1 |
| 2283 | Check if Number Has Equal Digit Count and Digit Value | Easy | Hash Table, String, Counting |
| 2287 | Rearrange Characters to Make Target String | Easy | Hash Table, String, Counting |
| 2341 | Maximum Number of Pairs in Array | Easy | Array, Hash Table, Counting |
| 2347 | Best Poker Hand | Easy | Array, Hash Table, Counting |
| 2351 | First Letter to Appear Twice | Easy | Bit Manipulation, Hash Table, String +1 |
| 2404 | Most Frequent Even Element | Easy | Array, Hash Table, Counting |
| 2423 | Remove Letter To Equalize Frequency | Easy | Hash Table, String, Counting |
| 2506 | Count Pairs Of Similar Strings | Easy | Bit Manipulation, Array, Hash Table +2 |
| 2529 | Maximum Count of Positive Integer and Negative Integer | Easy | Array, Binary Search, Counting |
| 2586 | Count the Number of Vowel Strings in Range | Easy | Array, String, Counting |
| 2748 | Number of Beautiful Pairs | Easy | Array, Hash Table, Math +2 |
| 2833 | Furthest Point From Origin | Easy | String, Counting |
| 2960 | Count Tested Devices After Test Operations | Easy | Array, Counting, Simulation |
| 1213 | Intersection of Three Sorted ArraysPremium | Easy | Array, Hash Table, Binary Search +1 |
Medium (65)
| # | Problem | Difficulty | Topics |
|---|---|---|---|
| 229 | Majority Element II | Medium | Array, Hash Table, Counting +1 |
| 299 | Bulls and Cows | Medium | Hash Table, String, Counting |
| 347 | Top K Frequent Elements | Medium | Array, Hash Table, Divide and Conquer +5 |
| 451 | Sort Characters By Frequency | Medium | Hash Table, String, Bucket Sort +3 |
| 621 | Task Scheduler | Medium | Greedy, Array, Hash Table +3 |
| 692 | Top K Frequent Words | Medium | Trie, Array, Hash Table +5 |
| 767 | Reorganize String | Medium | Greedy, Hash Table, String +3 |
| 811 | Subdomain Visit Count | Medium | Array, Hash Table, String +1 |
| 869 | Reordered Power of 2 | Medium | Hash Table, Math, Counting +2 |
| 900 | RLE Iterator | Medium | Design, Array, Counting +1 |
| 923 | 3Sum With Multiplicity | Medium | Array, Hash Table, Two Pointers +2 |
| 945 | Minimum Increment to Make Array Unique | Medium | Greedy, Array, Counting +1 |
| 1010 | Pairs of Songs With Total Durations Divisible by 60 | Medium | Array, Hash Table, Counting |
| 1054 | Distant Barcodes | Medium | Greedy, Array, Hash Table +3 |
| 1079 | Letter Tile Possibilities | Medium | Hash Table, String, Backtracking +1 |
| 1090 | Largest Values From Labels | Medium | Greedy, Array, Hash Table +2 |
| 1267 | Count Servers that Communicate | Medium | Depth-First Search, Breadth-First Search, Union Find +3 |
| 1347 | Minimum Number of Steps to Make Two Strings Anagram | Medium | Hash Table, String, Counting |
| 1366 | Rank Teams by Votes | Medium | Array, Hash Table, String +2 |
| 1400 | Construct K Palindrome Strings | Medium | Greedy, Hash Table, String +1 |
| 1419 | Minimum Number of Frogs Croaking | Medium | String, Counting |
| 1481 | Least Number of Unique Integers after K Removals | Medium | Greedy, Array, Hash Table +2 |
| 1497 | Check If Array Pairs Are Divisible by k | Medium | Array, Hash Table, Counting |
| 1519 | Number of Nodes in the Sub-Tree With the Same Label | Medium | Tree, Depth-First Search, Breadth-First Search +2 |
| 1657 | Determine if Two Strings Are Close | Medium | Hash Table, String, Counting +1 |
| 1726 | Tuple with Same Product | Medium | Array, Hash Table, Counting |
| 1737 | Change Minimum Characters to Satisfy One of Three Conditions | Medium | Hash Table, String, Counting +1 |
| 1775 | Equal Sum Arrays With Minimum Number of Operations | Medium | Greedy, Array, Hash Table +1 |
| 1781 | Sum of Beauty of All Substrings | Medium | Hash Table, String, Counting |
| 1814 | Count Nice Pairs in an Array | Medium | Array, Hash Table, Math +1 |
| 2001 | Number of Pairs of Interchangeable Rectangles | Medium | Array, Hash Table, Math +2 |
| 2013 | Detect Squares | Medium | Design, Array, Hash Table +1 |
| 2023 | Number of Pairs of Strings With Concatenation Equal to Target | Medium | Array, Hash Table, String +1 |
| 2029 | Stone Game IX | Medium | Greedy, Array, Math +2 |
| 2048 | Next Greater Numerically Balanced Number | Medium | Hash Table, Math, Backtracking +2 |
| 2131 | Longest Palindrome by Concatenating Two Letter Words | Medium | Greedy, Array, Hash Table +2 |
| 2150 | Find All Lonely Numbers in the Array | Medium | Array, Hash Table, Counting |
| 2170 | Minimum Operations to Make the Array Alternating | Medium | Greedy, Array, Hash Table +1 |
| 2182 | Construct String With Repeat Limit | Medium | Greedy, Hash Table, String +2 |
| 2186 | Minimum Number of Steps to Make Two Strings Anagram II | Medium | Hash Table, String, Counting |
| 2225 | Find Players With Zero or One Losses | Medium | Array, Hash Table, Counting +1 |
| 2244 | Minimum Rounds to Complete All Tasks | Medium | Greedy, Array, Hash Table +1 |
| 2275 | Largest Combination With Bitwise AND Greater Than Zero | Medium | Bit Manipulation, Array, Hash Table +1 |
| 2284 | Sender With Largest Word Count | Medium | Array, Hash Table, String +1 |
| 2364 | Count Number of Bad Pairs | Medium | Array, Hash Table, Math +1 |
| 2384 | Largest Palindromic Number | Medium | Greedy, Hash Table, String +1 |
| 2442 | Count Number of Distinct Integers After Reverse Operations | Medium | Array, Hash Table, Math +1 |
| 2453 | Destroy Sequential Targets | Medium | Array, Hash Table, Counting |
| 2526 | Find Consecutive Integers from a Data Stream | Medium | Design, Queue, Hash Table +2 |
| 2531 | Make Number of Distinct Characters Equal | Medium | Hash Table, String, Counting |
| 2856 | Minimum Array Length After Pair Removals | Medium | Greedy, Array, Hash Table +3 |
| 2870 | Minimum Number of Operations to Make Array Empty | Medium | Greedy, Array, Hash Table +1 |
| 2981 | Find Longest Special Substring That Occurs Thrice I | Medium | Hash Table, String, Binary Search +2 |
| 2982 | Find Longest Special Substring That Occurs Thrice II | Medium | Hash Table, String, Binary Search +2 |
| 1198 | Find Smallest Common Element in All RowsPremium | Medium | Array, Hash Table, Binary Search +2 |
| 1612 | Check If Two Expression Trees are EquivalentPremium | Medium | Tree, Depth-First Search, Hash Table +2 |
| 1940 | Longest Common Subsequence Between Sorted ArraysPremium | Medium | Array, Hash Table, Counting |
| 2067 | Number of Equal Count SubstringsPremium | Medium | Hash Table, String, Counting +1 |
| 2083 | Substrings That Begin and End With the Same LetterPremium | Medium | Hash Table, Math, String +2 |
| 2168 | Unique Substrings With Equal Digit FrequencyPremium | Medium | Hash Table, String, Counting +2 |
| 2268 | Minimum Number of KeypressesPremium | Medium | Greedy, Hash Table, String +2 |
| 2539 | Count the Number of Good SubsequencesPremium | Medium | Hash Table, Math, String +2 |
| 2782 | Number of Unique CategoriesPremium | Medium | Union Find, Counting, Interactive |
| 2950 | Number of Divisible SubstringsPremium | Medium | Hash Table, String, Counting +1 |
| 2955 | Number of Same-End SubstringsPremium | Medium | Array, Hash Table, String +2 |
Hard (14)
| # | Problem | Difficulty | Topics |
|---|---|---|---|
| 992 | Subarrays with K Different Integers | Hard | Array, Hash Table, Counting +1 |
| 1255 | Maximum Score Words Formed by Letters | Hard | Bit Manipulation, Array, Hash Table +5 |
| 1782 | Count Pairs Of Nodes | Hard | Graph, Array, Hash Table +4 |
| 1819 | Number of Different Subsequences GCDs | Hard | Array, Math, Counting +1 |
| 1857 | Largest Color Value in a Directed Graph | Hard | Graph, Topological Sort, Memoization +3 |
| 1994 | The Number of Good Subsets | Hard | Bit Manipulation, Array, Hash Table +5 |
| 2014 | Longest Subsequence Repeated k Times | Hard | Greedy, String, Backtracking +2 |
| 2025 | Maximum Number of Ways to Partition an Array | Hard | Array, Hash Table, Counting +2 |
| 2416 | Sum of Prefix Scores of Strings | Hard | Trie, Array, String +1 |
| 2499 | Minimum Total Cost to Make Arrays Unequal | Hard | Greedy, Array, Hash Table +1 |
| 2514 | Count Anagrams | Hard | Hash Table, Math, String +2 |
| 2547 | Minimum Cost to Split an Array | Hard | Array, Hash Table, Dynamic Programming +1 |
| 358 | Rearrange String k Distance ApartPremium | Hard | Greedy, Hash Table, String +3 |
| 1121 | Divide Array Into Increasing SequencesPremium | Hard | Array, Counting |
Keep exploring
All LeetCode topicsEvery tag with a page of its own, with the number of problems behind each.LeetCode patternsThe techniques themselves, with a Python template and complexity for each.Curated study listsBlind 75, NeetCode 150 and three more, grouped by pattern.Easy problemsStart here when the data structure is new rather than the problem.
Other topics
- Array1,569
- String672
- Hash Table588
- Math485
- Dynamic Programming481
- Sorting392
- Greedy346
- Depth-First Search289
- Binary Search253
- Database249
- Tree225
- Breadth-First Search223
- Matrix216
- Two Pointers201
- Bit Manipulation194
- Binary Tree174
- Heap (Priority Queue)163
- Prefix Sum157
- Stack157
- Simulation144
- Graph138
- Design122
- Sliding Window116
- Backtracking105
When the Counting problem arrives live
Stealth Interview is a desktop app for macOS and Windows. It reads the coding problem off your screen, returns a working solution with a step-by-step explanation and its time and space complexity, and transcribes what the interviewer is saying — while staying invisible to screen sharing.