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?

This problem was asked by Amazon.

Given a N by M matrix of numbers, print out the matrix in a clockwise spiral.

For example, given the following matrix:

[[1, 2, 3, 4, 5],
[6, 7, 8, 9, 10],
[11, 12, 13, 14, 15],
[16, 17, 18, 19, 20]]
You should print out the following:

1
2
3
4
5
10
15
20
19
18
17
16
11
6
7
8
9
14
13
12

This question was asked by Google.

Given an integer n and a list of integers l, write a function that randomly generates a number from 0 to n-1 that isn't in l (uniform).

This problem was asked by Lyft.

Given a list of integers and a number K, return which contiguous elements of the list sum to K.

For example, if the list is [1, 2, 3, 4, 5] and K is 9, then it should return [2, 3, 4], since 2 + 3 + 4 = 9.

This problem was asked by Google.

Given the head of a singly linked list, reverse it in-place.

This problem was asked by Google.

Given an array of integers, return a new array where each element in the new array is the number of smaller elements to the right of that element in the original input array.

For example, given the array [3, 4, 9, 6, 1], return [1, 1, 2, 1, 0], since:

There is 1 smaller element to the right of 3
There is 1 smaller element to the right of 4
There are 2 smaller elements to the right of 9
There is 1 smaller element to the right of 6
There are no smaller elements to the right of 1

This problem was asked by Google.

You are given an array of length n + 1 whose elements belong to the set {1, 2, ..., n}. By the pigeonhole principle, there must be a duplicate. Find it in linear time and space.

This problem was asked by Jane Street.

Given an arithmetic expression in Reverse Polish Notation, write a program to evaluate it.

The expression is given as a list of numbers and operands. For example: [5, 3, '+'] should return 5 + 3 = 8.

For example, [15, 7, 1, 1, '+', '-', '/', 3, '*', 2, 1, 1, '+', '+', '-'] should return 5, since it is equivalent to ((15 / (7 - (1 + 1))) * 3) - (2 + (1 + 1)) = 5.

You can assume the given expression is always valid.

This problem was asked by Square.

Given a list of words, return the shortest unique prefix of each word. For example, given the list:

dog
cat
apple
apricot
fish
Return the list:

d
c
app
apr
f

Given a 32-bit integer, return the number with its bits reversed.

For example, given the binary number 1111 0000 1111 0000 1111 0000 1111 0000, return 0000 1111 0000 1111 0000 1111 0000 1111.

This problem was asked by Slack.

You are given an N by M matrix of 0s and 1s. Starting from the top left corner, how many ways are there to reach the bottom right corner?

You can only move right and down. 0 represents an empty space while 1 represents a wall you cannot walk through.

For example, given the following matrix:

[[0, 0, 1],
[0, 0, 1],
[1, 0, 0]]
Return two, as there are only two ways to get to the bottom right:

Right, down, down, right
Down, right, down, right
The top left corner and bottom right corner will always be 0.

This problem was asked by Amazon.

Given a string, determine whether any permutation of it is a palindrome.

For example, carrace should return true, since it can be rearranged to form racecar, which is a palindrome. daily should return false, since there's no rearrangement that can form a palindrome.

This problem was asked by Facebook.

Given a positive integer n, find the smallest number of squared integers which sum to n.

For example, given n = 13, return 2 since 13 = 32 + 22 = 9 + 4.

Given n = 27, return 3 since 27 = 32 + 32 + 32 = 9 + 9 + 9.

This problem was asked by Amazon.

Implement a stack API using only a heap. A stack implements the following methods:

push(item), which adds an element to the stack
pop(), which removes and returns the most recently added element (or throws an error if there is nothing on the stack)
Recall that a heap has the following operations:

push(item), which adds a new key to the heap
pop(), which removes and returns the max value of the heap

This problem was asked by Triplebyte.

You are given n numbers as well as n probabilities that sum up to 1. Write a function to generate one of the numbers with its corresponding probability.

For example, given the numbers [1, 2, 3, 4] and probabilities [0.1, 0.5, 0.2, 0.2], your function should return 1 10% of the time, 2 50% of the time, and 3 and 4 20% of the time.

You can generate random numbers between 0 and 1 uniformly.

Given a 2-D matrix representing an image, a location of a pixel in the screen and a color C, replace the color of the given pixel and all adjacent same colored pixels with C.

For example, given the following matrix, and location pixel of (2, 2), and 'G' for green:

B B W
W W W
W W W
B B B
Becomes

B B G
G G G
G G G
B B B