Mock interviews

How a mock works

1. TimedAbout 3 minutes a question, with a visible clock you can pause.
2. Out loudAnswer as you would in the room. Nothing is recorded.
3. Follow-upsThen the follow-up questions an interviewer would ask.
4. RubricTick the key points you covered; that is your score.

Timed rounds (3)

Set a timer for the round, take the questions in order and say your approach out loud before you answer.

• Phone screen

  3 questions · 20 min

  One warm-up and one core problem, talking through the approach before coding.

  1. The first five minutes of a coding interview 3 min
  2. Do two sorted prices add up to a target? 8 min
  3. Do the brackets in a line balance? 9 min

• Coding round

  3 questions · 45 min

  Two problems of rising difficulty, with time at the end to test and state complexity.

  1. Longest streak of days within a budget 18 min
  2. Count the separate patches of land on a map 20 min
  3. Testing your own code in an interview 7 min

• Onsite loop round

  4 questions · 60 min

  One medium and one hard problem plus a follow-up discussion on trade-offs.

  1. The k-th biggest reading 15 min
  2. Fewest edits to turn one word into another 20 min
  3. Can every task finish, given its prerequisites? 18 min
  4. Heap, sort or quickselect? 7 min

Mock sets (34)

• Arrays, strings and hash maps mock 1

  Arrays, strings and hash maps · 5 questions · about 25 min

  • Group tile racks that hold the same letters
  • Count sales stretches that add up to a target
  • Longest run of consecutive ticket numbers
  • Find the value that fills more than half the list
  • Each shelf's product of all the other shelves

  Start this mock on the Arrays, strings and hash maps page

• Arrays, strings and hash maps mock 2

  Arrays, strings and hash maps · 5 questions · about 25 min

  • Shift a playlist k places to the right, in place
  • How a hash map keeps lookups fast on average
  • Spot the first repeated badge at the door
  • Two gift prices that hit the budget exactly
  • First order code that appears only once

  Start this mock on the Arrays, strings and hash maps page

• Two pointers mock 1

  Two pointers · 5 questions · about 25 min

  • Count weight triples that stay under a limit
  • Sort a tray of three token colours in one pass
  • When do two pointers work, and why?
  • Do two sorted prices add up to a target?
  • Push the empty parking slots to the end

  Start this mock on the Two pointers page

• Two pointers mock 2

  Two pointers · 5 questions · about 25 min

  • Does a banner read the same both ways?
  • Merge two sorted shelves into one
  • Remove repeated readings from a sorted log in place
  • Squared deviations in sorted order
  • Is a short route hidden inside a long one?

  Start this mock on the Two pointers page

• Sliding window mock 1

  Sliding window · 5 questions · about 25 min

  • Longest stretch of a log with no letter seen twice
  • Shortest run of orders that reaches a goal
  • Where does a rearranged pattern appear in a text?
  • Longest single-colour run after repainting k tiles
  • Longest stretch with at most two kinds

  Start this mock on the Sliding window page

• Sliding window mock 2

  Sliding window · 5 questions · about 25 min

  • Why a window with a nested loop is still O(n)
  • When a sliding window does not work
  • Busiest hour in each k-hour span
  • Longest streak of days within a budget
  • Best total over k consecutive days

  Start this mock on the Sliding window page

• Trees mock 1

  Trees · 5 questions · about 25 min

  • Is this a valid binary search tree?
  • Longest path between any two nodes
  • What you see from the right of a tree
  • The k-th smallest value in a search tree
  • Closest shared ancestor in a search tree

  Start this mock on the Trees page

• Trees mock 2

  Trees · 5 questions · about 25 min

  • Why search trees need to stay balanced
  • How trees are written on these pages
  • How deep is the deepest branch?
  • List a tree's values row by row
  • Are two trees identical?

  Start this mock on the Trees page

• Trees mock 3

  Trees · 5 questions · about 25 min

  • Does a tree mirror itself?
  • Swap every node's left and right children
  • Is every node's left and right height within one?
  • Does a root-to-leaf route add up to a target?
  • Pre-order, in-order, post-order and level order

  Start this mock on the Trees page

• Graphs mock 1

  Graphs · 5 questions · about 25 min

  • Count the separate patches of land on a map
  • Can every task finish, given its prerequisites?
  • Minutes until every item on a shelf spoils
  • How many separate groups does a network have?
  • The earliest valid task order

  Start this mock on the Graphs page

• Graphs mock 2

  Graphs · 5 questions · about 25 min

  • Seat guests at two tables with no feud at either
  • Which cable closes a loop?
  • Does a dependency graph contain a loop?
  • Fill in the water that does not touch the edge
  • Mark visited when you push or when you pop?

  Start this mock on the Graphs page

• Graphs mock 3

  Graphs · 5 questions · about 25 min

  • Why Dijkstra fails with negative weights
  • Union-find: how it works and when to reach for it
  • Fastest delivery time to every depot
  • Fewest one-letter changes between two code words
  • Cheapest way to cable every office together

  Start this mock on the Graphs page

• Graphs mock 4

  Graphs · 5 questions · about 25 min

  • Cells whose rain reaches both coasts
  • Fewest hops between two stations
  • Area of the biggest patch of land
  • Repaint the connected region of a picture
  • Adjacency list, adjacency matrix or edge list?

  Start this mock on the Graphs page

• Complexity analysis mock 1

  Complexity analysis · 5 questions · about 25 min

  • Amortised cost: cheap on average, sometimes slow
  • Which complexity fits which input size
  • Best, average and worst case
  • Working out the cost of recursion
  • How fast can sorting be?

  Start this mock on the Complexity analysis page

• Complexity analysis mock 2

  Complexity analysis · 5 questions · about 25 min

  • Raise a number to a huge power, modulo m
  • Big O in plain words
  • Reading the cost of loops
  • Where O(log n) comes from
  • Space complexity and the call stack

  Start this mock on the Complexity analysis page

• Stacks and queues mock 1

  Stacks and queues · 5 questions · about 25 min

  • How many days until a warmer day?
  • A stack that also reports its minimum
  • Evaluate an expression written in postfix order
  • Tidy a file path with . and .. parts
  • The monotonic stack pattern

  Start this mock on the Stacks and queues page

• Stacks and queues mock 2

  Stacks and queues · 5 questions · about 25 min

  • Recursion is a stack: converting to an explicit one
  • Largest poster that fits under a skyline
  • Do the brackets in a line balance?
  • Apply the backspaces in typed text
  • Build a queue out of two stacks

  Start this mock on the Stacks and queues page

• Searching sorted data mock 1

  Searching sorted data · 5 questions · about 25 min

  • Slowest reading pace that still finishes on time
  • Smallest value in a rotated sorted list
  • Find a value in a rotated sorted list
  • First and last position of a value in a sorted list
  • Smallest truck that delivers every parcel in d days

  Start this mock on the Searching sorted data page

• Searching sorted data mock 2

  Searching sorted data · 5 questions · about 25 min

  • Highest point of a climb-then-descent route
  • Binary search without off-by-one bugs
  • Binary search on the answer
  • Where would a new price go in a sorted list?
  • Whole-number square root without math.sqrt

  Start this mock on the Searching sorted data page

• Linked lists mock 1

  Linked lists · 5 questions · about 25 min

  • Drop the k-th link from the end
  • Fold a chain: first, last, second, second-last
  • Sum two numbers stored digit by digit
  • Does a chain read the same backwards?
  • How linked-list questions give you the chain

  Start this mock on the Linked lists page

• Linked lists mock 2

  Linked lists · 5 questions · about 25 min

  • Turn a chain of links around
  • Does the chain loop back on itself?
  • Weave two sorted chains into one
  • The placeholder head trick
  • When a chain beats a Python list

  Start this mock on the Linked lists page

• Heaps and priority queues mock 1

  Heaps and priority queues · 5 questions · about 25 min

  • Push values into a min-heap by hand
  • The k-th biggest reading
  • Merge many sorted feeds into one
  • The k depots nearest the warehouse
  • Fewest time slots when each job type needs a cooldown

  Start this mock on the Heaps and priority queues page

• Heaps and priority queues mock 2

  Heaps and priority queues · 5 questions · about 25 min

  • Cheapest way to join ropes into one
  • Heap, sort or quickselect?
  • Median after each new reading
  • Smash the two heaviest rocks until one is left
  • How a binary heap works

  Start this mock on the Heaps and priority queues page

• Backtracking mock 1

  Backtracking · 5 questions · about 25 min

  • Count the picks that hit an exact total
  • Every order of a playlist
  • Every balanced string of n bracket pairs
  • Trace a word through a letter grid
  • Cut a word into pieces that each read the same backwards

  Start this mock on the Backtracking page

• Backtracking mock 2

  Backtracking · 5 questions · about 25 min

  • Every distinct order when songs repeat
  • Pruning: making exponential searches fast enough
  • How big is the search space?
  • Count the ways to place queens that cannot attack
  • Every selection from a set of toppings

  Start this mock on the Backtracking page

• Dynamic programming mock 1

  Dynamic programming · 5 questions · about 25 min

  • Fewest coins to make an amount
  • Best haul without opening two neighbouring lockers
  • Best haul when the lockers form a ring
  • Ways to read a digit string as letters (1 = A … 26 = Z)
  • Can a string be split into dictionary words?

  Start this mock on the Dynamic programming page

• Dynamic programming mock 2

  Dynamic programming · 5 questions · about 25 min

  • Longest shared subsequence of two strings
  • Split items into two piles of equal weight
  • Number of ways to pay an amount with unlimited coins
  • Largest product of a contiguous run
  • Dynamic programming in four steps

  Start this mock on the Dynamic programming page

• Dynamic programming mock 3

  Dynamic programming · 5 questions · about 25 min

  • How to tell a problem needs DP
  • Fewest edits to turn one word into another
  • Longest rising sequence of picks from a list
  • Saving memory with rolling rows
  • Ways to cross n stepping stones, one or two at a time

  Start this mock on the Dynamic programming page

• Greedy choices and intervals mock 1

  Greedy choices and intervals · 5 questions · about 25 min

  • Fewest bookings to cancel so none clash
  • How many rooms do the bookings need at once?
  • Add a booking to a sorted list of separate blocks
  • Fewest hops to the last tile
  • Where to start a delivery loop so the van never runs dry

  Start this mock on the Greedy choices and intervals page

• Greedy choices and intervals mock 2

  Greedy choices and intervals · 5 questions · about 25 min

  • Cut a string into the most chunks so no letter crosses a cut
  • When does a greedy choice work?
  • Sweep-line thinking
  • Combine overlapping bookings into blocks
  • Can you hop to the last tile?

  Start this mock on the Greedy choices and intervals page

• Strings and tries mock 1

  Strings and tries · 5 questions · about 25 min

  • How many words start with a given prefix?
  • Longest stretch that reads the same backwards
  • Top three suggestions as you type
  • Look up words where '.' stands for any letter
  • Count the stretches that read the same backwards

  Start this mock on the Strings and tries page

• Strings and tries mock 2

  Strings and tries · 5 questions · about 25 min

  • Trie or hash set?
  • First place a pattern appears in a text
  • Rolling hashes for pattern search
  • Longest start shared by every word
  • What Python string operations cost

  Start this mock on the Strings and tries page

• Bits and maths mock 1

  Bits and maths · 5 questions · about 25 min

  • Bit tricks worth knowing
  • Working modulo 10⁹ + 7
  • The one value without a partner
  • Count the 1 bits in a number
  • 1-bit counts for every number from 0 to n

  Start this mock on the Bits and maths page

• Interview craft mock 1

  Interview craft · 5 questions · about 25 min

  • Spotting the pattern from the question
  • The first five minutes of a coding interview
  • Brute force first, then improve
  • Count position pairs whose values differ by exactly k
  • Talking while you code

  Start this mock on the Interview craft page