Programming interviews are like standard plays in professional sport - prepare accordingly. Don't let Programming Interview gotchas get you down!
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Coding interviews are nothing like software engineering jobs, they tend to be stressful and focus on the hardest parts of a software engineer's jobs. However, getting your core concepts right, with a lot of practice is the secret sauce to cracking the coding interview.
Pointers are the foundation of all hard interview problems, visualizing memory layout using pointers helps us understand what's really happening under the covers.
Practice is the key to understanding key pointer concepts, solve pointer problems by visualizing the memory layout.
Arrays are pointers at heart, work with them exactly like you would with pointers.
Pointers know how much space the data they point to occupies. Which means you can access all elements of an array with a single pointer, pointer arithmetic is pretty cool!
A whole bunch of practice with pointer problems. These should be easy because you will have the memory layout right there to help you visualize things.
Strings are character pointers which are equivalent to character arrays. Solve functions from the string.h library for practice dealing with pointers.
Pointers as arguments to functions have subtleties which need to be understood to use them correctly. Understand reassignment and modifications of pointers in a function and see how the original pointers are affected in the calling code.
Let's solve some harder problems from the string.h library, remember the little details like string termination, null inputs all matter in an interview!
Pointers just hold addresses of a memory location, which means we can have pointers to pointers to pointers. Sounds complicated? No worries, there are examples to help you understand these every step of the way.
Pointers to pointers requires a heightened conceptualization of memory layout. See detailed visuals on how pointer modification and reassignment work. User defined types or structs can also have pointers to them, memory layout and visualization of struct pointers.
Linked lists are favorite interview questions for software engineering roles. If you can work linked lists you're on your way to tackling more complicated problems. Understand the memory set up of linked lists and start with a few problems to gain confidence.
Get the length of a linked list, access the nth element in a list, and append an element to the end of the list - all this while handling null lists and other details.
For a simple concept, linked lists can get surprisingly tricky very quickly. Practice and practice again to gain mastery over linked list problems.
Use the linked list as a stack and implement pop, delete all the elements in a list - tricky memory freeing here, insert an element at a specified position and in a sorted list - these are similar but the edge cases differ.
Once you've actually solved all the examples we've got so far, you'll find that linked lists are fun! This class has a particularly difficult sample problem which will introduce you to the fast and slow pointers which traverse a linked list at different speeds. Useful for a lot of tricky maneuvering.
Append one list to another and split a list into two by using fast and slow pointers. This second problem is much, much harder than it seems.
By now you should be able to solve linked list problems in your sleep. Let's practice a few last ones to gain complete master over them.
Remove duplicates from a sorted list, move the first node from one list to another, merge two sorted lists and finally reverse a linked list.
Doubly linked lists lend themselves to fewer interesting problems. However its important to not overlook them, let's try deleting a node in a doubly linked list and see how to work with forward and reverse pointers.
Functions to get the nth bit of an integer and to set the nth bit of an integer. These are the building block functions and the concepts underlying these will be used for harder bit manipulation problems.
Print all the bits used to represent an integer from the most significant bit to the least significant. Learn some subtle details about the shift right (>>) with negative numbers!
Count the number of 1s in an integer, and learn a neat trick which allows you to do it in complexity O(number of 1s).
Reverse the bits in an integer. This pulls together a whole bunch of stuff from the last few problems. As in the case of hard problems, visualizing the process is key to solving this!
During coding interviews you might encounter questions which you can work out from first principles. You should be nailing these! Let's start with figuring out whether a string is a palindrome and finding all the points within a certain distance from another point.
Two more problems and detailed solutions. Play the game of life where every cell can change states from live to dead based on its neighbours.
Then move on to breaking a document into chunks to send down to a client subject to very specific constraints.
Run length encoding involves specifying the number of times a character is repeated in a string. Decoding run-length-encoded strings can be pretty tricky, let's find a solution for both.
If a number were represented by its digits, can you write code to add 2 numbers represented in this way? Let's walk through a solution and see if you can get this right.
Write code to check whether a Sudoku board is valid. This should work for both complete and incomplete boards. Sudoku is tricky and this has many conditions to check.
Lastly set up your own numeric system and then increment a number represented in that system by 1.
What is the performance of your code? How do you measure this? What is complexity and what is its relationship with performance?
The Big O notation is used to express complexity based on the size of the input specified for any algorithm. How is Big O expressed, how is it calculated and many examples to drive the concepts home!
A sorting algorithm is not just defined by its complexity, there are a whole bunch of other characteristics which can be used to determine which sorting algorithm is the right one for a system. Let's understand what these characteristics are and what are the trade offs we might make.
The simplest and most naive sorting algorithm.
Closely allied with selection sort is bubble sort. Its an adaptive sort with the same time complexity as selection sort.
Insertion sort is an improvement over both bubble sort and selection sort. Let's see how exactly it works and why it's preferred in many cases.
Shell sort builds on top of insertion sort, it improves the complexity of it's running time by partitioning the list in a clever way.
This belongs to a class of algorithms which uses divide and conquer to break the problem set into smaller pieces. This also makes a time-space trade off to get a faster running time.
Quick sort is the sort of choice for developers of programming libraries. Let's see what makes it so attractive.
Binary search is a pretty nifty way to search through a sorted list in O(Log N) time. It's also an interview favorite so make sure you understand it well!
Recursion is pretty hard at the beginning. Let's look at an example of reversing a string and see how we can use recursion to solve the problem. Visualize the input and every step and see how the magic of recursion works.
We've already seen and understood binary search. This is a perfect first problem to tackle using recursion. Make sure you try it yourself first before seeing the solutions in the class.
A classic problem which can be solved recursively. Try out a few smaller sets and see how their subsets look to find patterns.
Binary trees lend themselves to problems which have really beautiful recursive solutions. The problem may seem hard but the solutions end up being simple. Checking for whether 2 trees are the same is one such problem.
Paint fill allows you to color regions on screen while using drawing software. Implement a recursive solution to paint fill a region on the display screen.
Say you were given all the tasks needed to build a complete car. These tasks may depend on one another. Set up a data structure to represent a task and it's dependencies and write code to build a car.
An anagram of a word is simply a word with the letters of the original word rearranged. This is complicated but lends itself well to a recursive solution.
There can be several paths out of a maze. Help a rat placed anywhere in maze to find it's way out.
Another classic problem with an elegant recursive solution.
The stack is a very simple and easy to understand data structure. However it lies underneath many complicated real world problems and is incredibly useful.
Let's build a stack for real using Java. It'll have all the operations we're interested in - push, pop, peek, size etc. It can hold any data type, it's a generic class.
Problems which use stacks as a part of their solutions are very common in programming interviews. Matching parenthesis to check for well formed expressions is a classic interview question - let's solve this using the stack we're already implemented.
Another interview question implemented. You have space available but your processing needs to be very fast indeed. How would you keep track of the minimum element of a stack as it changes?
Depth first traversal can be of 3 types based on the order in which the node is processed relative to it's left and right sub-trees.
In-order traversal processes the left subtree, then the node itself and then it's right sub trees. Post-order traversal processes the node *after* it's left and right subtrees.
The algorithms are all remarkably similar and very easy once you use recursion.
Loonycorn is us, Janani Ravi and Vitthal Srinivasan. Between us, we have studied at Stanford, been admitted to IIM Ahmedabad and have spent years working in tech, in the Bay Area, New York, Singapore and Bangalore.
Janani: 7 years at Google (New York, Singapore); Studied at Stanford; also worked at Flipkart and Microsoft
Vitthal: Also Google (Singapore) and studied at Stanford; Flipkart, Credit Suisse and INSEAD too
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