1Numbers & Math

18 problems

The arithmetic warm-ups every fresher is asked first: digits, factors, primes, and number tricks.

01Sum of Two NumbersAdd two integers and return the result.mathSolved 02Maximum of Two NumbersReturn the larger of two integers.comparisonSolved 03Maximum of Three NumbersReturn the largest of three integers.comparisonSolved 04Even or OddDecide whether a number is even or odd.moduloSolved 05Sign of a NumberReport whether a number is positive, negative, or zero.conditionalsSolved 06Leap YearCheck whether a year is a leap year.moduloSolved 07Sum from 1 to NAdd up every integer from 1 to N.mathSolved 08Factorial of a NumberCompute n! = 1·2·…·n.mathSolved 09Count DigitsCount how many digits a number has.digit dpSolved 10Sum of DigitsAdd up all the digits of a number.digit dpSolved 11Reverse a NumberReverse the digits of an integer.digit dpSolved 12Palindrome NumberCheck whether a number reads the same backwards.digit dpSolved 13Prime NumberCheck whether a number is prime.mathSolved 14Nth Fibonacci NumberReturn the Nth number in the Fibonacci sequence.iterationSolved 15GCD of Two NumbersFind the greatest common divisor of two integers.euclidSolved 16LCM of Two NumbersFind the least common multiple of two integers.euclidSolved 17Power of a NumberCompute x raised to the power n.mathSolved 18Armstrong NumberCheck whether a number equals the sum of its digits cubed.digit dpSolved

2Arrays

22 problems

Single-pass scans over a list: max, min, sum, search, reverse, and the classic counting tricks.

19Maximum in an ArrayFind the largest element in an array.linear scanSolved 20Minimum in an ArrayFind the smallest element in an array.linear scanSolved 21Sum of Array ElementsAdd up every element of an array.linear scanSolved 22Average of an ArrayCompute the mean of the array elements.linear scanSolved 23Count Even and OddCount even and odd numbers in an array.linear scanSolved 24Linear SearchFind the index of a target value by scanning.linear scanSolved 25Reverse an ArrayReverse the order of elements in place.two pointersSolved 26Second Largest ElementFind the second largest value in one pass.linear scanSolved 27Count OccurrencesCount how many times a value appears.linear scanSolved 28Check If Array Is SortedDecide if the array is in non-decreasing order.linear scanSolved 29Sum of Even NumbersAdd only the even elements of an array.linear scanSolved 30Find a DuplicateFind an element that appears more than once.hash setSolved 31Move Zeros to EndPush all zeros to the end, keep order.two pointersSolved 32Left Rotate by OneShift every element one position left.arraySolved 33Count Positive and NegativeCount positives, negatives, and zeros.linear scanSolved 34Missing NumberFind the missing number in 1..n.mathSolved 35Merge Two Sorted ArraysMerge two sorted arrays into one.two pointersSolved 36Separate Odd and EvenPut odds and evens into separate lists.linear scanSolved 37Max Consecutive OnesLongest run of 1s in a binary array.running countSolved 38Count Distinct ElementsCount how many unique values exist.hash setSolved 39Sum of Min and MaxAdd the smallest and largest elements.linear scanSolved 40Frequency of ElementsCount how often each value appears.hash mapSolved

3Strings

22 problems

Character-by-character processing: reverse, count, compare, and the fresher-favourite palindrome and anagram checks.

41Reverse a StringReverse the characters of a string.two pointersSolved 42Palindrome StringCheck whether a string reads the same backwards.two pointersSolved 43Count Vowels and ConsonantsCount vowels and consonants in a string.linear scanSolved 44Length of a StringCount the characters in a string without library help.linear scanSolved 45Convert to UppercaseConvert every lowercase letter to uppercase.asciiSolved 46Convert to LowercaseConvert every uppercase letter to lowercase.asciiSolved 47Count WordsCount the words in a sentence.linear scanSolved 48Valid AnagramCheck whether two strings are anagrams.frequencySolved 49Character FrequencyCount how often each character appears.frequencySolved 50Remove SpacesBuild a copy of the string without spaces.linear scanSolved 51Count a CharacterCount occurrences of one character.linear scanSolved 52First Non-Repeating CharacterFind the first character that appears once.frequencySolved 53Toggle CaseSwap the case of every letter.asciiSolved 54Check Strings EqualCompare two strings character by character.linear scanSolved 55Longest WordReturn the longest word in a sentence.linear scanSolved 56Remove Duplicate CharactersKeep only the first occurrence of each char.hash setSolved 57Capitalize Each WordUppercase the first letter of every word.asciiSolved 58Count Upper and LowerCount uppercase and lowercase letters.asciiSolved 59Replace a CharacterReplace every occurrence of one char with another.linear scanSolved 60Reverse Words in a SentenceReverse the order of words.stringSolved 61Check SubstringDecide whether one string contains another.stringSolved 62Count Uppercase LettersCount the capital letters in a string.asciiSolved

4Loops & Patterns

12 problems

Pure loop control: number series, the FizzBuzz classic, and the star/number patterns asked in campus rounds.

63Print 1 to NPrint every integer from 1 to N.loopsSolved 64Print Even NumbersPrint all even numbers up to N.loopsSolved 65Multiplication TablePrint the multiplication table of a number.loopsSolved 66Sum of SquaresAdd 1² + 2² + … + n².loopsSolved 67FizzBuzzPrint Fizz, Buzz, or FizzBuzz for 1..n.moduloSolved 68Right Triangle Star PatternPrint a right-angled triangle of stars.nested loopsSolved 69Square Star PatternPrint an n×n square of stars.nested loopsSolved 70Pyramid Star PatternPrint a centred pyramid of stars.nested loopsSolved 71Number TrianglePrint a triangle of increasing numbers.nested loopsSolved 72Fibonacci SeriesPrint the first N Fibonacci numbers.iterationSolved 73Multiples of a NumberPrint the first N multiples of a number.loopsSolved 74Primes up to NPrint all prime numbers up to N.mathSolved

5Searching & Sorting

8 problems

The textbook algorithms every interviewer expects a fresher to code from memory.

75Binary SearchFind a target in a sorted array in O(log n).binary searchSolved 76Bubble SortSort an array by repeatedly swapping neighbours.sortingSolved 77Selection SortSort by repeatedly selecting the minimum.sortingSolved 78Insertion SortSort by inserting each element into place.sortingSolved 79Count Greater Than XCount elements strictly greater than X.linear scanSolved 80Sort 0s and 1sSort a binary array in one pass.two pointersSolved 81Kth Smallest ElementFind the Kth smallest value.sortingSolved 82First and Last OccurrenceFind the first and last index of a value.linear scanSolved

6Recursion

8 problems

The same warm-ups, re-derived recursively — base case, recursive case, and the call stack.

83Factorial (Recursion)Compute n! recursively.recursionSolved 84Fibonacci (Recursion)Compute the Nth Fibonacci recursively.recursionSolved 85Sum 1 to N (Recursion)Add 1..N recursively.recursionSolved 86Reverse String (Recursion)Reverse a string recursively.recursionSolved 87Power (Recursion)Compute xⁿ recursively.recursionSolved 88Sum of Digits (Recursion)Add the digits of a number recursively.recursionSolved 89Print N to 1 (Recursion)Print N down to 1 recursively.recursionSolved 90GCD (Recursion)Find the GCD recursively with Euclid.recursionSolved

7Bit Tricks & Misc

10 problems

A gentle first look at binary, XOR tricks, and a couple of must-know conversions.

91Power of TwoCheck whether a number is a power of two.bit manipulationSolved 92Count Set BitsCount the 1-bits in a number's binary form.bit manipulationSolved 93Check If a Bit Is SetTest whether the k-th bit is 1.bit manipulationSolved 94Swap Without a TempSwap two numbers without a third variable.bit manipulationSolved 95Odd-Occurring NumberFind the number that appears an odd number of times.xorSolved 96Decimal to BinaryConvert a decimal number to binary.bit manipulationSolved 97Binary to DecimalConvert a binary string to decimal.bit manipulationSolved 98Even Using BitwiseCheck even/odd with the AND operator.bit manipulationSolved 99Single NumberFind the element that appears once using XOR.xorSolved 100Celsius to FahrenheitConvert a temperature between scales.mathSolved

Basic 100 — DSA for Freshers

Get the most out of these 100

All 100 problems

1Numbers & Math

2Arrays

3Strings

4Loops & Patterns

5Searching & Sorting

6Recursion

7Bit Tricks & Misc

Level up from the basics

Basic 100 — DSA for Freshers

Get the most out of these 100

All 100 problems

1Numbers & Math

2Arrays

3Strings

4Loops & Patterns

5Searching & Sorting

6Recursion

7Bit Tricks & Misc

Level up from the basics