Practice

10 interview-style problems.

Full statement, worked examples, and runnable starter code. Open any problem in the playground and the test fixtures are pre-loaded — no setup, no sign-up. Free to run in 100+ languages.

01easy
Binary Search
Given a sorted array and a target, return the target's index, or -1 if absent. O(log N).
arraysearch
02easy
Fibonacci
Read N and print the Nth Fibonacci number. F(0) = 0, F(1) = 1, F(N) = F(N-1) + F(N-2).
mathdynamic-programming
03easy
FizzBuzz
Read N, print numbers 1..N — but multiples of 3 become "Fizz", multiples of 5 become "Buzz", multiples of both become "FizzBuzz".
mathloop
04easy
Merge Two Sorted Arrays
Given two sorted arrays, print a single merged sorted output.
arraytwo-pointer
05easy
Palindrome Check
Read a string, print "true" if it is a palindrome (case-insensitive, alphanumerics only), else "false".
stringtwo-pointer
06easy
Reverse a Linked List
Reverse a singly linked list. Iterative (O(1) extra space) or recursive (O(n) stack).
linked-listpointer
07easy
Reverse String
Reverse the input string. In-place is preferred where the language allows.
stringtwo-pointer
08easy
Two Sum
Given an array of integers and a target, return the indices of the two numbers that add up to the target.
arrayhashmap
09easy
Valid Anagram
Given two strings, print "true" if one is a permutation of the other.
stringhashmapsorting
10easy
Valid Parentheses
Given a string containing only (){}[], determine whether every opener has its matching closer, correctly nested.
stackstring