PHP LeetCode Problems: How to Solve and What to Study

PHP on LeetCode: An Unexpected but Powerful Ally in Interview Preparation

When you hear "LeetCode", Java, Python, or C++ usually come to mind. PHP seems like an outsider in this list. But if you write PHP and want to grow — from a simple "scripter" to an engineer who designs high-load systems — you absolutely need to go through LeetCode in PHP.

Why? Because LeetCode is not about syntax. It's about algorithmic thinking. And PHP, for all its web-specific nature, gives you the same tools for working with arrays, strings, trees, and graphs. Just with slightly different syntax and some "pitfalls".

In this article, we'll break down which topics you need to brush up on, how to write efficient PHP code for LeetCode, and show real solution examples. Let's go!

Why Does a PHP Developer Need LeetCode?

Many think: "I'm a PHP backend developer, knowing Laravel and SQL is enough." This is a trap. In interviews at large companies (and not only), you'll be given a problem on binary search or dynamic programming. And if you can't solve such problems in PHP, you won't get hired, even if you're a Symfony guru.

LeetCode trains:

\n
• The ability to analyze algorithm complexity (Big O). Without this, you won't be able to optimize queries or write efficient queues.
\n
• Working with non-standard data structures — stacks, queues, hash tables, trees. In PHP, they are not as obvious as in Java.
\n
• Code cleanliness — you learn to write short, clear functions without unnecessary crutches.
\n

And yes, PHP on LeetCode is not a link to documentation. It's a full-fledged language for solutions. The main thing is to know its features.

What Do You Need to Study Before Starting?

1. Arrays — PHP's Main Tool

PHP doesn't have classic lists or hash tables — it's all arrays. They can be both indexed and associative. This is powerful, but also dangerous: you can accidentally mix key types.

What's important:

\n
• array_map(), array_filter(), array_reduce() — your best friends.
\n
• Sorting: sort(), asort(), ksort(). Remember that sort() resets keys.
\n
• Checking key existence: isset() and array_key_exists() are not the same.
\n

2. Strings and Regular Expressions

PHP is famous for its string functions. LeetCode often has problems on palindromes, anagrams, character counting. Use str_split(), count_chars(), preg_match() (but don't overuse it — in an interview they might ask you to do it without regular expressions).

3. Recursion and Stack

PHP doesn't have a built-in stack (SplStack exists but is rarely used). Most often, a stack is emulated with an array using array_push() and array_pop(). Recursion also works, but watch the depth — the default limit is 256 calls.

4. Classes and Objects

LeetCode often gives problems where you need to implement a class (stack, queue, LRU cache). In PHP, this is done through classes with private/protected properties. Don't forget about __construct() and type hinting.

Typical LeetCode Problems for PHP

Let's break down three popular problems — from easy to medium difficulty. The code will be in PHP, with explanations.

Problem 1: Two Sum (Easy)

Condition: Given an array of numbers and a target sum. Find the indices of two numbers that add up to the target.

Solution using a hash table (associative array):

function twoSum(array $nums, int $target): array {\n    $map = []; // key — number, value — index\n    \n    foreach ($nums as $i => $