Java LeetCode Problems: From Beginner to Interview

LeetCode and Java: Your Key to FAANG and Beyond

If you are learning Java and want to get into a major IT company (or just level up your skills), then LeetCode is a platform you cannot ignore. Tens of thousands of developers solve Java problems on LeetCode daily to sharpen their algorithmic thinking and prepare for technical interviews.

But there's a catch: simply "solving problems" is not enough. You need to understand how to solve them, which data structures to use, and what to pay attention to specifically in Java. In this article, I'll tell you everything: from choosing your first problems to analyzing real code. Let's go!

Why Java Specifically for LeetCode?

Java is one of the most popular languages for passing interviews. Why? Firstly, it is strictly typed, which forces you to think about data types. Secondly, Java has a rich standard library: ArrayList, HashMap, PriorityQueue and other collections are your secret weapon. Thirdly, Java developers are in demand in large corporations where LeetCode-style problems are a mandatory selection stage.

But there are also pitfalls: Java is verbose. What can be written in one line in Python might take 5-10 lines in Java. This is not a problem if you are used to the syntax. The main thing is not to be afraid of the code volume.

Where Should a Beginner Start with Java on LeetCode?

Many make the mistake of jumping straight into complex problems (Hard) and getting disappointed. Start small. Here is my proven plan:

1. Master Basic Data Structures in Java

Before opening LeetCode, make sure you can confidently write:

• Arrays — the absolute basics. int[] arr = new int[10];
• Lists — ArrayList<Integer> list = new ArrayList<>();
• Hash Tables — HashMap<String, Integer> map = new HashMap<>();
• Stacks and Queues — Deque<Integer> stack = new ArrayDeque<>();
• Strings — methods charAt(), substring(), StringBuilder.

2. Solve Problems by Topic

LeetCode allows you to filter problems by tags. Start with Easy problems on topics:

• Two Pointers
• Sliding Window
• Hash Table
• String Manipulation

Example: the Two Sum problem is a classic. Here is its solution in Java:

import java.util.HashMap;import java.util.Map;

public class TwoSum {    public int[] twoSum(int[] nums, int target) {        Map<Integer, Integer> map = new HashMap<>();        for (int i = 0; i < nums.length; i++) {            int complement = target - nums[i];            if (map.containsKey(complement)) {                return new int[]{map.get(complement), i};            }            map.put(nums[i], i);        }        throw new IllegalArgumentException("No two sum solution");    }}

Notice: we use HashMap for O(1) lookup. This is a standard technique.

Top 5 Types of LeetCode Problems for a Java Developer

To prepare for an interview, you need to know the "gentleman's set" of topics. Here they are:

1. Arrays and Strings

The most common problems. Learn to work with substrings, prefixes, suffixes. A useful pattern is Sliding Window.

Example problem: "Longest Substring Without Repeating Characters".

2. Linked Lists

Java does not have a built-in LinkedList (well, there is LinkedList, but on LeetCode they usually provide their own node). Know how to reverse a list, delete elements, and find cycles.

public class ListNode {    int val;    ListNode next;