# Archive for the ‘Easy’ Category

## Introduction

In this lesson we will talk about a way of returning values from threads, more precisely we will talk about std::future which is a mechanism that C++ offers in order to perform asynchronous tasks and query for the result in the future.
A future represents an asynchronous task, i.e. an operation that runs in parallel to the current thread and which the latter can wait (if it needs to) until the former is ready.
You can use a future all the time you need a thread to wait for a one-off event to happen. The thread can check the status of the asynchronous operation by periodically polling the future while still performing other tasks, or it can just wait for the future to become ready.

## Introduction

In the previous lesson we have seen how data can be protected using mutex. We now know how to make threads do their work concurrently without messing around with shared resources and data. But sometimes we need to synchronize their actions in time, meaning that we might want a thread t1 to wait until a certain condition is true before allowing it to continue its execution.

This lesson discusses the tools that we can use to achieve such behavior efficiently using condition variables.

# Programming Interview Question

## Question: given two string write a method to decide if one is a permutation of the other.

This is a common question that could be easily solved if we know in advance which is the size of the alphabeth. A straightforward approach could be to sort both strings and then compare them. This approach has complexity  due to sorting. We can do better and lower the complexity to  if we reason as follows: given a boolean sequence of the same size of the alphabet  initially false initialized.  Then what happen if  for each char we found in the  string  we negate the value of the correnspoding bool in the sequence? We end up having the  at position  if the char  appeared an odd number of times and  otherwise. Now if the other string  is a permutation of the first one, we all agree that for each char in  it contains the same number of occurrences. This means that if we apply the same process as before on the boolean sequence using as input the string  each bool is being negated an even number of times and this means that its value would be the same as the beginning of the process (all ). So if the sequence does not contains any true value, than it means that the strings contains the same elements i.e. they are permutation of each other.

Example:

1. "abbccd" ,  "cbdabc" , 
2. Apply negation using s
• 
3. Apply negation using v
• 

A possible C++ implementation is shown here

# Programming Intervew Question: Unique Characters in a String

##### Question: implement an algorithm to determine if a string has all unique characters

The idea behing this (fairly easy) question is that whenever we found that any chars is repeated at least twice we should return false. In order to do that we have to look at each char we are loloking for a solution which complexity is at least . We are free to use any support data structure. is the size of the ASCII charset.

Programming Question: Given an integer, compute its parity(easy)
The parity of an integer is true iff number of set bits (1) is odd, false otherwise. Example: has 5 set bits and hence its parity is true.

# Programming Question: given a 2D array, print it in spiral fashion (easy)

Given a 2D array print it in spiral fashion as shown in the picture.