#1 | Balanced parentheses algorithm with tail-call recursion optimisationBalanced parentheses algorithm with tail-call recursion | #### Free |

#2 | Check if an array is a palindromeCheck if an array is a palindrome | #### Free |

#3 | Find an unpaired number in an arrayFind an unpaired number in an array | #### Free |

#4 | Count binary gap size of a number using tail recursionCount binary gap size of a number using tail recursion | #### Free |

#5 | Find maximum potential profit from an array of stock priceFind maximum potential profit from an array of stock price | #### Free |

#6 | Fizz Buzz in purely functional immutable ScalaFizz Buzz in purely functional immutable Scala | #### Free |

#7 | Binary search in a rotated sorted arrayBinary search in a rotated sorted array | #### Solution: US$1.99#### Free test cases & hints |

#8 | Find combinations adding up to N (unique)Find combinations adding up to N (unique) | #### Solution: US$1.99#### Free test cases & hints |

#9 | Find the index of a substring ('indexOf')Find the index of a substring ('indexOf') | #### Solution: US$1.99#### Free test cases & hints |

#10 | Reverse bits of an integerReverse bits of an integer | #### Solution: US$1.99#### Free test cases & hints |

#11 | Find k closest elements to a value in a sorted ArrayFind k closest elements to a value in a sorted Array | #### Solution: US$1.99#### Free test cases & hints |

#12 | Reshape a matrixReshape a matrix | #### Solution: US$1.99#### Free test cases & hints |

#13 | Remove duplicates from a sorted list (Sliding)Remove duplicates from a sorted list (Sliding) | #### Solution: US$1.99#### Free test cases & hints |

#14 | Remove duplicates from a sorted list (state machine)Remove duplicates from a sorted list (state machine) | #### Solution: US$1.99#### Free test cases & hints |

#15 | Find combinations adding up to N (non-unique)Find combinations adding up to N (non-unique) | #### Solution: US$1.99#### Free test cases & hints |

#16 | Compute single-digit sum of digitsCompute single-digit sum of digits | #### Solution: US$1.99#### Free test cases & hints |

#17 | Add numbers without using addition (plus sign)Add numbers without using addition (plus sign) | #### Solution: US$1.99#### Free test cases & hints |

#18 | Tic Tac Toe MinMax solveTic Tac Toe MinMax solve | #### Solution: US$1.99#### Free test cases & hints |

#19 | Compute nth row of Pascal's triangleCompute nth row of Pascal's triangle | #### Solution: US$1.99#### Free test cases & hints |

#20 | Print Alphabet DiamondPrint Alphabet Diamond | #### Solution: US$1.99#### Free test cases & hints |

#21 | Reverse a String's words efficientlyReverse a String's words efficiently | #### Solution: US$1.99#### Free test cases & hints |

#22 | Monitor success rate of a process that may failMonitor success rate of a process that may fail | #### Solution: US$1.99#### Free test cases & hints |

#23 | QuickSelect Selection Algorithm (kth smallest item/order statistic)QuickSelect Selection Algorithm | #### Solution: US$1.99#### Free test cases & hints |

#24 | Rotate a matrix by 90 degrees clockwiseRotate a matrix by 90 degrees | #### Solution: US$1.99#### Free test cases & hints |

#25 | Read a matrix as a spiralRead a matrix as a spiral | #### Solution: US$1.99#### Free test cases & hints |

#26 | Count number of contiguous countries by colorsCount number of contiguous countries by colors | #### Solution: US$1.99#### Free test cases & hints |

#27 | Length of the longest common substringLength of the longest common substring | #### Solution: US$1.99#### Free test cases & hints |

#28 | Find minimum missing positive number in a sequenceFind minimum missing positive number in a sequence | #### Solution: US$1.99#### Free test cases & hints |

#29 | Binary search a generic ArrayBinary search a generic Array | #### Solution: US$1.99#### Free test cases & hints |

#30 | Run-length encoding (RLE) DecoderRun-length encoding (RLE) Decoder | #### Solution: US$1.99#### Free test cases & hints |

#31 | Run-length encoding (RLE) EncoderRun-length encoding (RLE) Encoder | #### Solution: US$1.99#### Free test cases & hints |

#32 | Find the contiguous slice with the minimum averageFind the contiguous slice with the minimum average | #### Solution: US$1.99#### Free test cases & hints |

#33 | Tic Tac Toe board checkTic Tac Toe board check | #### Solution: US$1.99#### Free test cases & hints |

#34 | Find the minimum absolute difference of two partitionsFind the minimum absolute difference of two partitions | #### Solution: US$1.99#### Free test cases & hints |

#35 | Compute a Roman numeral for an Integer, and vice-versaCompute a Roman numeral for an Integer, and vice-versa | #### Solution: US$1.99#### Free test cases & hints |

#36 | Quick Sort sorting algorithm in pure immutable ScalaQuick Sort sorting algorithm in pure immutable Scala | #### Solution: US$1.99#### Free test cases & hints |

#37 | Fibonacci in purely functional immutable ScalaFibonacci in purely functional immutable Scala | #### Solution: US$1.99#### Free test cases & hints |

#38 | In a range of numbers, count the numbers divisible by a specific integerCount numbers divisible by another | #### Solution: US$1.99#### Free test cases & hints |

#39 | Is an Array a permutation?Is an array a permutation? | #### Solution: US$1.99#### Free test cases & hints |

#40 | Merge Sort: stack-safe, tail-recursive, in pure immutable Scala, N-wayMerge Sort: stack-safe, tail-recursive | #### Solution: US$1.99#### Free test cases & hints |

#41 | Closest pair of coordinates in a 2D planeClosest pair of coordinates in a 2D plane | #### Solution: US$1.99#### Free test cases & hints |

#42 | Counting inversions of a sequence (array) using a Merge SortCounting inversions of a sequence | #### Solution: US$1.99#### Free test cases & hints |

#43 | Rotate Array right in pure-functional Scala - using an unusual immutable efficient approachRotate Array right in pure-functional Scala | #### Solution: US$1.99#### Free test cases & hints |

#44 | Count factors/divisors of an integerCount factors of a number | #### Solution: US$1.99#### Free test cases & hints |

#45 | Merge Sort: in pure immutable ScalaMerge Sort: in pure immutable Scala | #### Solution: US$1.99#### Free test cases & hints |

#46 | Longest increasing sub-sequence lengthLongest increasing sub-sequence length | #### Solution: US$1.99#### Free test cases & hints |

#47 | Matching parentheses algorithm with foldLeft and a state machineMatching parentheses algorithm with a state machine | #### Solution: US$1.99#### Free test cases & hints |

#48 | Traverse a tree Breadth-First, immutablyTraverse a tree Breadth-First, immutably | #### Solution: US$1.99#### Free test cases & hints |

#49 | Count number of changes (manipulations) needed to make an anagram with foldLeft and a MultiSetCount number of changes for making an anagram | #### Solution: US$1.99#### Free test cases & hints |

#50 | Check if a number is a palindromeCheck if a number is a palindrome | #### Solution: US$1.99#### Free test cases & hints |

#51 | Compute maximum sum of subarray (Kadane's algorithm)Compute maximum sum of subarray (Kadane's algorithm) | #### Solution: US$1.99#### Free test cases & hints |

#52 | Check if a String is a palindromeCheck if a String is a palindrome | #### Solution: US$1.99#### Free test cases & hints |

#53 | Find sub-array with the maximum sumFind sub-array with the maximum sum | #### Solution: US$1.99#### Free test cases & hints |

#54 | Traverse a tree Depth-FirstTraverse a tree Depth-First | #### Solution: US$1.99#### Free test cases & hints |

#55 | Compute the length of longest valid parenthesesCompute the length of longest valid parentheses | #### Available 13th May |

#56 | Single-elimination tournament treeSingle-elimination tournament tree | #### Available 20th May |

#57 | Make a queue using stacks (Lists in Scala)Make a queue using stacks (Lists in Scala) | #### Available 27th May |

#58 | Make a queue using MapsMake a queue using Maps | #### Available 3rd June |

#59 | Remove duplicates from an unsorted listRemove duplicates from an unsorted list | #### Available 10th June |

#60 | Count passing carsCount passing cars | #### Available 17th June |

#61 | Pure-functional double linked listPure-functional double linked list | #### Available 24th June |

#62 | Check if a directed graph has cyclesCheck if a directed graph has cycles | #### Available 1st July |

#63 | Longest common prefix of stringsLongest common prefix of strings | #### Available 8th July |

#64 | Check a directed graph has a routing between two nodes (depth-first search)Check a directed graph has a routing between two nodes | #### Available 15th July |

#65 | Make a binary search treeMake a binary search tree | #### Available 22nd July |

#66 | Check a binary tree is a search treeCheck a binary tree is a search tree | #### Available 29th July |

#67 | Find height of binary treeFind height of binary tree | #### Available 5th August |

#68 | Least-recently used cache (LRU)Least-recently used cache (LRU) | #### Available 12th August |

#69 | Reverse first n elements of a queueReverse first n elements of a queue | #### Available 19th August |

#70 | Compute keypad possibilitiesCompute keypad possibilities | #### Available 26th August |

#71 | Check a binary tree is balancedCheck a binary tree is balanced | #### Available 2nd September |

#72 | Establish execution order from dependenciesEstablish execution order from dependencies | #### Available 9th September |

#73 | Count pairs of a given expected sumCount pairs of a given expected sum | #### Available 16th September |

#74 | Compute modulo of an exponent without exponentiationCompute modulo of an exponent without exponentiation | #### Available 23rd September |

#75 | Binary heapBinary heap | #### Available 30th September |

#76 | Binomial heapBinomial heap | #### Available 7th October |

#77 | Game of LifeGame of Life | #### Available 14th October |

#78 | Leaky Bucket Rate LimiterLeaky Bucket Rate Limiter | #### Available 21st October |

#79 | Token Bucket Rate LimiterToken Bucket Rate Limiter | #### Available 28th October |

#80 | Fixed Window Rate LimiterFixed Window Rate Limiter | #### Available 4th November |

#81 | Sliding Window Rate LimiterSliding Window Rate Limiter | #### Available 11th November |

#82 | Compute the steps to transform an anagram only using swapsCompute the steps to transform an anagram only using swaps | #### Available 18th November |