Java Collection Methods Useful for LeetCode Interviews

A curated list of high-impact Java Collection methods to solve LeetCode problems efficiently

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Introduction

Here are the Top Java Collection Methods that are super helpful for LeetCode-style problems, especially in coding interviews where performance and brevity matter:

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1. Map.getOrDefault(key, defaultValue)

• Returns the value for the specified key if present in the map; otherwise, returns the provided default value.

• Ideal for counting frequencies or caching intermediate results.

map.put(num, map.getOrDefault(num, 0) + 1);

Used in: Two Sum, Group Anagrams, Top K Frequent Elements etc

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2. Set.add() Return Value

• The add() method attempts to insert an element into a Set. It returns a boolean indicating whether the set was modified (i.e., whether the element was not already present). Detects duplicates in O(1) time.

if (!set.add(num)) {
    // Duplicate detected
}

Used in: Contains Duplicate, Valid Sudoku etc.

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3. List.subList(fromIndex, toIndex)

• Returns a view of the portion of the list between fromIndex (inclusive) and toIndex (exclusive). It does not create a new list—just a window into the original.

• Changes to the sublist affect the original list, and vice versa.

• An efficient way to handle sliding windows or divide-and-conquer.

List<Integer> sub = list.subList(i, i + k);

Used in: Sliding Window problems, Backtracking etc

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4. Deque for Monotonic Queue / Sliding Window

• Deque (Double-Ended Queue) is a powerful tool for sliding window problems, especially when you need to efficiently find the maximum or minimum in a window in O(n) time using a Monotonic Queue.

• Efficient for problems involving windowed max/min.

Deque<Integer> deque = new ArrayDeque<>();

Used in: Sliding Window Maximum

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5. PriorityQueue (Min-Heap / Max-Heap)

• PriorityQueue is a binary heap implementation in Java used for problems requiring access to the smallest or largest elements efficiently.

• Great for maintaining the top k elements.

PriorityQueue<Integer> minHeap = new PriorityQueue<>();
PriorityQueue<Integer> maxHeap = new PriorityQueue<>(Collections.reverseOrder());

PriorityQueue<Integer> minHeap = new PriorityQueue<>();
minHeap.offer(3);
minHeap.offer(1);
minHeap.offer(5);
System.out.println(minHeap.poll()); // 1 (smallest)

Used in: Kth Largest Element, Median from Data Stream

6. List.removeIf(predicate)

• Removes all elements from a List that match the given predicate (a condition). It's a concise and efficient way to filter a list in-place.

list.removeIf(element -> condition);

Bonus Helpers

• Map.computeIfAbsent(): One-liner to group or initialize maps. Efficiently initialize or fetch a default value if the key doesn’t exist, without verbose null-checking.

• Collections.frequency(): Quick frequency check in lists. Counts how many times a specific element appears in a collection (typically a List).

• Collections.reverse(list)Reverse in-place.

Conclusion

Mastering the right Java Collection methods can make your LeetCode solutions cleaner, faster, and more interview-friendly. Whether we are counting frequencies, handling duplicates, processing sliding windows, or managing heaps, these methods offer concise and powerful abstractions that save both time and boilerplate.

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Still not convinced? Check out the details of the past work

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