Best Coding Challenges for Beginners (Ordered by Concept)

The best coding challenges for beginners are the fifteen below, worked in this order: strings, arrays, hash maps, math, recursion, stacks, linked lists, and two pointers. Each concept builds on the one before it, so the order matters more than the platform you solve them on. Every challenge here is a real interview question, appears on every major judge, and can be solved in under 30 lines in any language.

For each challenge you get one sentence on what it asks, one on what it teaches, and the name of the pattern it belongs to. A pattern is a reusable approach that solves a whole family of problems, and naming it is how you recognize the next problem of the same kind. There are no full solutions on this page, because the point is to write them yourself.

Strings

Reverse a string: given a string, return it with the characters in the opposite order. It teaches indexing from both ends and the difference between a string and a character array. Pattern: Two Pointers.

Palindrome check: decide whether a string reads the same forward and backward, ignoring case and non-letter characters. It teaches input cleaning before the main logic. Pattern: Two Pointers.

Valid anagram: decide whether two strings contain the same letters with the same counts. It teaches counting characters in a table instead of sorting. Pattern: Hash Map counting.

Arrays

Two sum: find two numbers in an array that add to a target and return their positions. It teaches trading memory for time, since a map lookup replaces a second loop. Pattern: Hash Map.

Maximum subarray: find the contiguous run of numbers with the largest sum. It teaches keeping a running best while scanning once, which is the idea behind many dynamic programming solutions. Pattern: Kadane's algorithm.

Move zeroes: move every zero to the end of an array while keeping the other numbers in order, without making a new array. It teaches in-place writes with a slow index and a fast index. Pattern: Two Pointers.

Hash maps

First non-repeating character: return the first character in a string that appears exactly once. It teaches the two-pass approach: count everything, then scan for the first count of one. Pattern: Hash Map counting.

Group anagrams: group a list of words so that words with the same letters end up together. It teaches choosing a key, here the sorted word, so that equal groups map to one entry. Pattern: Hash Map with a computed key.

Math

FizzBuzz: print the numbers from 1 to n, replacing multiples of 3 with Fizz, multiples of 5 with Buzz, and multiples of both with FizzBuzz. It teaches the order of conditions, since the "both" case must be tested first. Pattern: none, plain control flow.

Prime check: decide whether a number is prime by testing divisors. It teaches stopping at the square root, which cuts a million checks to a thousand for a number near one million. Pattern: Math.

Recursion

Factorial: compute n multiplied by every smaller positive integer. It teaches the shape of every recursive function: a base case and a smaller call. Pattern: Recursion.

Fibonacci: return the nth number in the sequence where each number is the sum of the two before it. It teaches why plain recursion repeats work, and it is the usual first example of memoization, which means saving each result so it is computed once. Pattern: Recursion, then Dynamic Programming.