This is a complete coverage playground repo I use as template for my tdd projects and I maintain updated solving popular coding challenges once in a while.
In this problem you will implement a feature for a text editor to find errors in the usage of brackets in the code.
Your friend is making a text editor for programmers. He is currently working on a feature that will find errors in the usage of different types of brackets. Code can contain any brackets from the set []{}(), where the opening brackets are [,{, and ( and the closing brackets corresponding to them are ],}, and ).
For convenience, the text editor should not only inform the user that there is an error in the usage of brackets, but also point to the exact place in the code with the problematic bracket. First priority is to find the first unmatched closing bracket which either doesn’t have an opening bracket before it, like ] in ](), or closes the wrong opening bracket, like } in ()[}. If there are no such mistakes, then it should find the first unmatched opening bracket without the corresponding closing bracket after it, like ( in {}([]. If there are no mistakes, text editor should inform the user that the usage of brackets is correct.
Apart from the brackets, code can contain big and small latin letters, digits and punctuation marks. More formally, all brackets in the code should be divided into pairs of matching brackets, such that in each pair the opening bracket goes before the closing bracket, and for any two pairs of brackets either one of them is nested inside another one as in (foo[bar]) or they are separate as in f(a,b)-g[c]. The bracket [ corresponds to the bracket ], { corresponds to }, and ( corresponds to ).
The implementation has to be the most efficient possible and be able to handle large input sizes.
Solution:
This problem was recently asked by Google.
Given a list of numbers and a number k, return whether any two numbers from the list add up to k.
For example, given [10, 15, 3, 7] and k of 17, return true since 10 + 7 is 17.
Bonus: Can you do this in one pass?
Solution:
This problem was asked by Uber.
Given an array of integers, return a new array such that each element at index i of the new array is the product of all the numbers in the original array except the one at i.
For example, if our input was [1, 2, 3, 4, 5], the expected output would be [120, 60, 40, 30, 24]. If our input was [3, 2, 1], the expected output would be [2, 3, 6].
Follow-up: what if you can't use division?
Solution:
This problem was asked by Google.
Given the root to a binary tree, implement serialize(root), which serializes the tree into a string, and deserialize(s), which deserializes the string back into the tree.
For example, given the following Node class
class Node:
def __init__(self, val, left=None, right=None):
self.val = val
self.left = left
self.right = rightThe following test should pass:
node = Node('root', Node('left', Node('left.left')), Node('right'))
assert deserialize(serialize(node)).left.left.val == 'left.left'I decided not to take shortcuts (using custom serializer for pickle or json), but instead implement a tailored syntax. It's not too complex and allows for a more clean representation:
root
left
left.left
None
None
None
right
None
None
Solution:
This problem was asked by Stripe.
Given an array of integers, find the first missing positive integer in linear time and constant space. In other words, find the lowest positive integer that does not exist in the array. The array can contain duplicates and negative numbers as well.
For example, the input [3, 4, -1, 1] should give 2. The input [1, 2, 0] should give 3.
You can modify the input array in-place.
Solution:
This problem was asked by Jane Street.
cons(a, b) constructs a pair, and car(pair) and cdr(pair) returns the first and last element of that pair. For example, car(cons(3, 4)) returns 3, and cdr(cons(3, 4)) returns 4.
Given this implementation of cons:
def cons(a, b):
def pair(f):
return f(a, b)
return pairImplement car and cdr.
Solution:
This problem was asked by Google.
An XOR linked list is a more memory efficient doubly linked list. Instead of each node holding next and prev fields, it holds a field named both, which is an XOR of the next node and the previous node. Implement an XOR linked list; it has an add(element) which adds the element to the end, and a get(index) which returns the node at index.
If using a language that has no pointers (such as Python)(sic), you can assume you have access to get_pointer and dereference_pointer functions that converts between nodes and memory addresses(sic).
Notes:
-
Actually python has support for low-level memory operations, I implemented them without problem and requiring only a minimal hack; gc will free any non referenced object. The solution is to attach a self-reference to each node. The tradeof is having to delete this self-reference on removing the node, but it's way cleaner than messing with how gc operates or stoping gc altogheter.
-
I am not implementing the interfaces as fuctions but as methods.
-
List.addis not Pythonic, I called itList.append -
I added the
List.__iter__method to allow coversion to list to better test. -
There is a iterative and a recursive implementation of traversing in this implementation, this is on purpose; I wanted to try both approaches.
Solution:
This problem was asked by Facebook.
Given the mapping a = 1, b = 2, ... z = 26, and an encoded message, count the number of ways it can be decoded.
For example, the message '111' would give 3, since it could be decoded as 'aaa', 'ka', and 'ak'.
You can assume that the messages are decodable. For example, '001' is not allowed.
Solution:
This problem was asked by Google.
A unival tree (which stands for "universal value") is a tree where all nodes under it have the same value.
Given the root to a binary tree, count the number of unival subtrees.
For example, the following tree has 5 unival subtrees:
0
/ \
1 0
/ \
1 0
/ \
1 1
Solution:
This problem was asked by Airbnb. Difficulty: Hard
Given a list of integers, write a function that returns the largest sum of non-adjacent numbers. Numbers can be 0 or negative.
For example, [2, 4, 6, 2, 5] should return 13, since we pick 2, 6, and 5. [5, 1, 1, 5] should return 10, since we pick 5 and 5.
Follow-up: Can you do this in O(N) time and constant space?
Solution:
This problem was asked by Apple. Difficulty: Medium
Implement a job scheduler which takes in a function f and an integer n, and calls f
after n milliseconds.
N.B.: I guess this is more of a challenge in other languages..., still, I decided to keep the high level implementation in Python because this in one of the most overseenmodules in the standar library.
Solution:
This problem was asked by Amazon. Difficulty: Hard
There exists a staircase with N steps, and you can climb up either 1 or 2 steps at a time. Given N, write a function that returns the number of unique ways you can climb the staircase. The order of the steps matters.
For example, if N is 4, then there are 5 unique ways:
- 1, 1, 1, 1
- 2, 1, 1
- 1, 2, 1
- 1, 1, 2
- 2, 2
What if, instead of being able to climb 1 or 2 steps at a time, you could climb any number from a set of positive integers X? For example, if X = {1, 3, 5}, you could climb 1, 3, or 5 steps at a time.
Solution:
This problem was asked by Twitter. Difficulty: Medium
Implement an autocomplete system. That is, given a query string s and a set ofall possible query strings, return all strings in the set that have s as aprefix. For example, given the query string de and the set of strings [dog, deer, deal], return [deer, deal].
Hint: Try preprocessing the dictionary into a more efficient data structure tospeed up queries.
Solution:
This problem was asked by Facebook. Difficulty: Medium
Given a stream of elements too large to store in memory, pick a random elementfrom the stream with uniform probability.
Solution:
This problem was asked by Google. Difficulty: Medium
The area of a circle is defined as πr^2. Estimate π to 3 decimal places using a Monte Carlo method.
I increased speed using non-overlaping diagonals to define the points. (based on prime numbers)
Solution:
This problem was asked by Palantir. Difficulty: Medium
Write an algorithm to justify text. Given a sequence of words and an integerline length k, return a list of strings which represents each line, fully justified.
More specifically, you should have as many words as possible in each line. Thereshould be at least one space between each word. Pad extra spaces when necessary so that each line has exactly length k. Spaces should be distributed as equally as possible, with the extra spaces, if any, distributed starting from the left. If you can only fit one word on a line, then you should pad the right-hand sidewith spaces. Each word is guaranteed not to be longer than k.
For example: words = ["the", "quick", "brown", "fox", "jumps","over", "the", "lazy", "dog"] k = 16 you should return the following: [ "the quick brown", # 1 extra space on the left "fox jumps over", # 2 extra spaces distributed evenly "the lazy dog", # 4 extra spaces distributed evenly ]
Solution:
This problem was asked by Facebook. Difficulty: Medium
You are given an array of non-negative integers that represents a two-dimensional elevation map where each element is unit-width wall and the integer is the height. Suppose it will rain and all spots between two walls get filled up.
Compute how many units of water remain trapped on the map in O(N) time and O(1) space.
For example: - given the input [2, 1, 2], we can hold 1 unit of water in the middle. - given the input [3, 0, 1, 3, 0, 5], we can hold 3 units in the first index, 2 in the second, and 3 in the fourth index (we cannot hold 5 since it would run off to the left), so we can trap 8 units of water.
I added a stress test case fixture to ensure compliance in large arrays.
Solution:
This problem was asked by Google. Difficulty: Easy
The edit distance between two strings refers to the minimum number of character insertions, deletions, and substitutions required to change one string to the other.
For example, the edit distance between “kitten” and “sitting” is three: substitute the “k” for “s”, substitute the “e” for “i”, and append a “g”.
Given two strings, compute the edit distance between them.
Solution:
This problem was asked by Amazon. Difficulty: Easy
Run-length encoding is a fast and simple method of encoding strings. The basic idea is to represent repeated successive characters as a single count and character.
For example, the string "AAAABBBCCDAA" would be encoded as"4A3B2C1D2A".
Implement run-length encoding and decoding. You can assume the string to be encoded have no digits and consists solely of alphabetic characters. You can assume the string to be decoded is valid.
Solution:
This problem was asked by Facebook. Difficulty: Easy
Given a string of round, curly, and square open and closing brackets, return whether the brackets are balanced (well-formed).
For example, given the string "([])", you should return true. Given the string "([)]" or "((()", you should return false.
Solution:
This problem was asked by Jane Street. Difficulty: Hard
Suppose you are given a table of currency exchange rates, represented as a 2D array. Determine whether there is a possible arbitrage: that is, whether there is some sequence of trades you can make, starting with some amount A of any currency, so that you can end up with some amount greater than A of that currency. There are no transaction costs and you can trade fractional quantities.
Solution:
This problem was asked by Microsoft. Difficulty: Easy
Compute the running median of a sequence of numbers. That is, given a stream of numbers, print out the median of the list so far on each new element.
Recall that the median of an even-numbered list is the average of the two middle numbers.
For example, given the sequence [2, 1, 5, 7, 2, 0, 5], your algorithm should print out:
2
1.5
2
3.5
2
2
2
N.B.: Of course I am not printing, like it were the 90s. As well, the interface for the heaps is a bit of an overkill, but I think it really made the solution easier to read.
Solution:
This problem was asked by Facebook. Difficulty: Hard
Implement regular expression matching with the following special characters:
- "." (period) which matches any single character
- "*" (asterisk) which matches zero or more of the preceding element
That is, implement a function that takes in a string and a valid regular expression and returns whether or not the string matches the regular expression.
For example, given the regular expression "ra." and the string "ray", your function should return true. The same regular expression on the string "raymond" should return False.
Given the regular expression ".*at" and the string "chat", your function should return true. The same regular expression on the string "chats" should return False.
Solution:
This problem was asked by Google. Difficulty: Easy
You are given an M by N matrix consisting of booleans that represents a board. Each True boolean represents a wall. Each False boolean represents a tile you can walk on.
Given this matrix, a start coordinate, and an end coordinate, return the minimum number of steps required to reach the end coordinate from the start. If there is no possible path, then return None. You can move up, left, down, and right. You cannot move through walls. You cannot wrap around the edges of the board.
For example, given the following board:
[[0, 0, 0, 0],
[1, 1, 0, 1],
[0, 0, 0, 0],
[0, 0, 0, 0]]and start = (3, 0) (bottom left) and end = (0, 0) (top left), the minimum number of steps required to reach the end is 7, since we would need to go through (1,2) because there is a wall everywhere else on the second row.
Solution:
Difficulty: Easy
This problem was asked by Google.Given two strings A and B, return whether or not A can be shifted some number of times to get B. For example, if A is abcde and B is cdeab, return true. If A is abc and B is acb, return false.
Solution:
This problem was recently asked by Google. Difficulty: Easy
Given a list of numbers and a number k, return whether any two numbers from thelist add up to k.For example, given [10, 15, 3, 7] and k of 17, return true since 10 + 7 is 17.Bonus: Can you do this in one pass?
Solution: