Algorithm Explained: Get the max items you can purchase with a fixed budget

Problem statement: Given a fixed budget and a list of item prices. What is the max number of items you can purchase? You can only buy each item once. Note: This is the Mark and Toys problem from HackerRank. Example: You’re given $10 and a list of items to choose from: A cool coffee cup … Read more

C# – Check if a string contains any substring from a list

There are many different scenarios where you might want to check a string against a list of substrings. Perhaps you’re dealing with messy exception handling and have to compare the exception message against a list of known error messages to determine if the error is transient or not. When you need to check a string … Read more

C# – Pad a 2D array on all sides

Padding a 2D array on all sides means adding new rows on the top and bottom, new columns on the left and right, and then copying the original elements to the center of the padded array. It looks like this: There are two approaches for copying the elements. You can either copy individual items in … Read more

Algorithm Explained: Sum two big integers the hard way

Problem statement: Sum two big integers that are passed in as strings. Return the sum as a string. In other words, implement the following method: Constraint: Don’t use the built-in BigInteger class (note: this is the name in C# and may have a different name in other languages). Do it the hard way instead. If … Read more

C# – Join strings with a separator, ignoring nulls and empty strings

Normally when you want to join strings using a separator, you’d use string.Join(). However, the problem with string.Join() is it doesn’t ignore nulls or empty strings. Take a look at the following examples: If you want to filter out nulls and empty strings, you can filter the list of strings yourself and pass it into … Read more

C# – Remove a set of characters from a string

The fastest and simplest way to remove a set of characters from a string is to use StringBuilder + List<char>, like this: I compared this with three other approaches. I did 100,000 iterations with a string with 2500 characters and a list of 15 characters to remove. This StringBuilder approach is almost 2x faster than … Read more

Algorithm Explained: Determine if two strings have a substring in common

Problem statement: Given two strings, determine if they have a substring in common. Example: Do “hello world” and “world” have a substring in common? Yes, they both have the substring “world”. Approach What are all the substrings of string? The string “word” is four characters long. It contains 10 substrings between length 1 and 4. … Read more

Solving whiteboard problems to prepare for coding interviews

Companies like to ask you to solve whiteboard problems during coding interviews. If it’s an in-person interview, they’ll ask you to solve the problem on a physical whiteboard. If it’s a remote interview, they’ll either have you solve the problem on a website like HackerRank, or they’ll ask you the question in a “digital whiteboard” … Read more