LeetCode Problems by Category

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Difference array

Difference arrays are not super common but are important to know as it is very difficult to derive on the spot in an interview. This method can be used when the input is a 2D array, where input[i] is in the format [left, right, value], or some equivalent form. The story behind the problem will usually be something along the lines of "between left and right there is value of something". Let's look at an example.

LC 1094. Car Pooling

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Hash table

LC 1. Two Sum

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Prefix sum

If a problem's input is an array comprised of numeric elements and you find yourself needing to calculate multiple subarray sums, then consider building a prefix sum. But be mindful that a prefix sum costs $O(n)$ space to build — if the prefix sum is used in a "sliding" manner, then be on the lookout for space optimization possibilities by using a running sum and deducting that from the total sum for each iteration.

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Sliding window

If a problem has explicit constraints such as

sum greater than or less than k
limits on what is contained (such as maximum of k unique elements or no duplicates allowed)

and/or asks for

minimum or maximum length
minimum or maximum sum
number of subarrays/substrings

then think about a sliding window. Note that not all problems with these characteristics should be solved with a sliding window, and not all sliding window problems have these characteristics. These characteristics should only be used as a general guideline.

Clever endpoint trick

Fixed-width

LC 643. Maximum Average Subarray I

Variable-width

LC 1004. Max Consecutive Ones III

Stacks

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Two pointers

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SQL

Useful Reference

LC 180. Consecutive Numbers: Contains a detailed tutorial on gaps and islands problems and how to effectively use ROW_NUMBER() to identify where gaps occur.
LC 185. Department Top Three Salaries: Contains a detailed tutorial on correlated subqueries.

Summer 2021

Best problems

Good problems

Revisit

Beginner problems

CASE statements

Correlated subquery

Dates and timestamps

DELETE

LC 196. Delete Duplicate Emails

DISTINCT

EXISTS and NOT EXISTS

Gaps and islands

IN and NOT IN

Indicator variables

Joins

Cross-joins

LEFT JOIN

Non-equi-joins

RIGHT JOIN

LC 1159. Market Analysis II

Self-joins

Median

NOT EXISTS (see EXISTS)

NOT IN (see IN)

NULL values and effective management

Pivoting

Result set to multiple rows

Recursive CTEs

Regular expressions

Stored procedures

LC 177. Nth Highest Salary

UNION and UNION ALL

UPDATE

LC 627. Swap Salary

Window functions

Named windows

AVG()

COUNT()

DENSE_RANK()

LAG()

LC 1709. Biggest Window Between Visits

LEAD()

MAX()

MIN()

PERCENTILE_CONT()

LC 571. Find Median Given Frequency of Numbers

RANK()

ROW_NUMBER()

SUM()

With GROUP BY (ORDER BY aggregate)

LC 1341. Movie Rating

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Difference array​

E​

F​

G​

H​

Hash table​

I​

J​

K​

L​

M​

N​

O​

P​

Prefix sum​

Q​

R​

S​

Sliding window​

Clever endpoint trick​

Fixed-width​

Variable-width​

Stacks​

T​

Two pointers​

U​

V​

W​

X​

Y​

Z​

SQL​

Summer 2021​

Best problems​

Good problems​

Revisit​

Beginner problems​

CASE statements​

Correlated subquery​

Dates and timestamps​

DELETE​

DISTINCT​

EXISTS and NOT EXISTS​

Gaps and islands​

IN and NOT IN​

Indicator variables​

Joins​

Cross-joins​

LEFT JOIN​

Non-equi-joins​

RIGHT JOIN​

Self-joins​

Median​

NOT EXISTS (see EXISTS)​

NOT IN (see IN)​

NULL values and effective management​

Pivoting​

Result set to multiple rows​

Recursive CTEs​

Regular expressions​

Stored procedures​

UNION and UNION ALL​

UPDATE​

Window functions​

Named windows​

AVG()​

COUNT()​

DENSE_RANK()​

LAG()​

LEAD()​

MAX()​

MIN()​

PERCENTILE_CONT()​

RANK()​

ROW_NUMBER()​

SUM()​

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