2. Java Basics for DSA
Arrays, Strings, StringBuilder and Collections — the Java toolbox every later topic uses.
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9easy
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Understand the topic
Before patterns, you need to be fast with Java's toolbox: fixed-size arrays (int[]), Strings (immutable), StringBuilder (for building text), and Collections like ArrayList, HashMap and HashSet.
These problems each practise one tool. Pay attention to the small Java details interviewers notice: arrays can't grow, String comparison uses equals(), and StringBuilder avoids creating a new string on every +.
| # | Problem | Pattern | Level | ||
|---|---|---|---|---|---|
| 1 | Concatenation of Array #1929 | Array basics | Easy | LeetCode ↗ | |
| 2 | Shuffle the Array #1470 | Index arithmetic | Easy | LeetCode ↗ | |
| 3 | Fizz Buzz #412 | Strings & List | Easy | LeetCode ↗ | |
| 4 | Reverse String #344 | Two pointers | Easy | LeetCode ↗ | |
| 5 | Defanging an IP Address #1108 | StringBuilder | Easy | LeetCode ↗ | |
| 6 | Maximum Number of Words Found in Sentences #2114 | String scanning | Easy | LeetCode ↗ | |
| 7 | Kids With the Greatest Number of Candies #1431 | Array & List | Easy | LeetCode ↗ | |
| 8 | Shuffle String #1528 | char[] basics | Easy | LeetCode ↗ | |
| 9 | Count Items Matching a Rule #1773 | List of lists | Easy | LeetCode ↗ | |
| 10 | Insert Delete GetRandom O(1) #380 | ArrayList + HashMap | Medium | LeetCode ↗ |
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Work top to bottom — each list goes from easy to hard. Try a problem for at least 15 minutes before opening a hint. Ticks are saved in your browser.