Math for Technical Interviews

How much math do I need to know for technical interviews?

The short answer is about high school-level math.

Computer science is often associated with math, and some universities even place their computer science department under the math faculty. However, the reality is that you usually only need high school math for most of the software engineering interviews and day-to-day software engineering jobs.

But I have seen an xxx question on LeetCode that needs a yyy math trick

LeetCode has 2000+ questions, mainly user submitted. Having a particular question in the vast question bank doesn't mean very much. What really matters is whether the question is asked in an actual interview. If you look at the top questions companies ask, the questions that require advanced math tricks or knowledge are rarely asked. Questions that require knowing a particular math trick or fact are a knowledge test, and interviews are supposed to test your coding and problem-solving skills, not specific math knowledge.

What if I'm so unlucky that I got asked a tricky math question?

At most companies, candidates' performance ratings are reviewed by engineers other than the interviewers. And if a question is considered too difficult or off-topic, that round will be considered to carry less significance and assigned less weight in the final decision. So don't sweat about not knowing advanced math.

What if I don't even remember high school math?

You can learn it in one hour. Let's go through them now!

Understanding Number Bases

Before diving into logarithms, it's important to understand the concept of "bases" in mathematics. A base determines the number of unique digits and the value each position in a numeral system represents.

Base 10: We humans naturally count using the decimal (base 10) system, which has ten unique digits: 0 through 9. Think about how you count: after reaching the number 9, we reset to two digits, starting at 10. This pattern continues, with each position from the right representing an increasing power of 10.

For example, in the number 352:

• 2 is in the ones place (which is 10 ^ 0)
• 5 is in the tens place (which is 10 ^ 1)
• 3 is in the hundreds place (which is 10 ^ 2)

Transition to Base 2 in Computer Science: In contrast, computers operate using the binary (base 2) system. This system has only two unique digits: 0 and 1. The reasons for this binary nature are rooted in the on-off, true-false electronic logic of computer circuits. In the binary system, each position from the right represents an increasing power of 2.

In the binary number 1011:

• The rightmost 1 is in the "ones" place (which is 2 ^ 0),
• The next 1 is in the "twos" place (which is 2 ^ 1),
• The 0 indicates no value in the "fours" place (which is 2 ^ 2),
• The leftmost 1 is in the "eights" place (which is 2 ^ 3).

Introduction to Logarithms

What is a Logarithm? A logarithm answers the question: To what power must we raise a certain base to get a number? In simpler terms, it's the inverse of an exponential function.

Exponential Examples with Base 2:

2^2 means 2 multiplied by itself once: 2 * 2 = 4

2^3 means 2 multiplied by itself twice: 2 * 2 * 2 = 8

2^4 means 2 multiplied by itself thrice: 2 * 2 * 2 * 2 = 16

Understanding Logarithms with Base 2: Given a number, the logarithm tells us how many times we need to multiply 2 to obtain that number.

log(8) = 3 means we need three 2's multiplied together to get 8: 2 * 2 * 2

log(16) = 4 means we need four 2's multiplied together to get 16: 2 * 2 * 2 * 2

Alternatively, the logarithm tells us how many times we can divide a number by 2 until we reach 1:

8 / 2 / 2 / 2 = 1 - Dividing 8 by 2 three times gives 1, so log(8) = 3

16 / 2 / 2 / 2 / 2 = 1 - Dividing 16 by 2 four times gives 1, so log(16) = 4

Logarithms, especially with base 2, are fundamental in computer science because many computational problems instinctively split themselves in half.

Permutations and factorial

Sets and Sequences:

Set: A collection of distinct items, referred to as "elements", where the order of the items does not matter. E.g., {a, b}.

Permutation: A specific arrangement or ordering of the elements of a set. In permutations, the order is crucial. For the set {a, b}, we have two permutations: [a, b] and [b, a].

The following figure shows all the permutations of (a, b, c).

Counting permutations

Imagine arranging three letters: a, b, and c.

For the first position, you have 3 choices (a, b, or c). Once you've chosen the first letter, only 2 remain for the second position. For the third and final position, only 1 letter is left.

This gives a total of 3 * 2 * 1 permutations, or 6 possible sequences: [a, b, c], [a, c, b], [b, a, c], [b, c, a], [c, a, b], and [c, b, a].

We can generalize this idea to count the number of permutations for a set of size n.

For the first position, we have n choices. Once the first position has been fixed, there's n - 1 choices for the second position. Then, we have n - 2 choices for the third position. We'll keep making choices until only 1 letter is left.

From this, we can get that the number of permutations for a set of size n is n * (n-1) * (n-2)... 1. This is called factorial of n, denoted by n!. For example, 5! = 5 * 4 * 3 * 2 * 1 = 120. This means there are 120 ways to arrange 5 letters in a row.

Understanding Subsets

What is a Subset? A subset of a set A is another set that contains only the elements which are also present in A. For instance, the set {1, 3, 9} is a subset of {1, 2, 3, 5, 6, 7, 9} because every element of the former is present in the latter.

How Many Subsets Can a Set Have? When we look at the elements of a set, for each element, we have a choice:

• Include it in the subset
• Exclude it from the subset This gives two possibilities for each element.

Here's a figure to illustrate this. Think of it like switches: each element has a switch that can either be "ON" (included in the subset) or "OFF" (excluded from the subset).

• With one element (or switch), we have 2 ^ 1 or 2 possible states.
• With two elements, we have 2 ^ 2 or 4 possible states.
• With three elements, we'll get 2 ^ 3 or 8 possible states.

Expanding on this idea, if a set has n elements, then there will be 2 ^ n subsets. Note that these 2 ^ n subsets include the empty subset (where no elements are chosen) and the original set itself (where all elements are chosen).

Arithmetic sequence

An arithmetic sequence is a sequence of numbers such that the difference between the consecutive terms is constant. For example,

1 2 3 4 5 is an arithmetic sequence because the difference between consecutive numbers is 1.

1 3 5 7 9 is an arithmetic sequence because the difference between consecutive numbers is 2.

1 2 4 is NOT an arithmetic sequence because 2 - 1 = 1 (first difference) but 4 - 2 = 2 (second difference). Here, the differences between consecutives are different.

Sum of an arithmetic sequence

The sum of an arithmetic sequence is (first_element + last_element) * number_of_element / 2. Here's the animated proof from Wikipedia if you are interested.

For example:

sum([1,2,3,4,5]) = (1 + 5) * 5 / 2 = 15

sum([1,3,5,7,9]) = (1 + 9) * 5 / 2 = 25

Because last_element = first_element + difference * (number_of_element - 1) , the sum can be expressed as (2 * first_element + difference * (number_of_elements - 1)) * number_of_elements / 2). In big O complexity analysis, we drop the constant terms (first_element, difference and -1), so this really becomes O(n^2).

When is arithmetic sequence useful [for my interviews]?