Java Collection Methods Useful for LeetCode Interviews
A curated list of high-impact Java Collection methods to solve LeetCode problems efficiently
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
2. Set.add() Return Value
The
add()method attempts to insert an element into aSet. 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.
3. List.subList(fromIndex, toIndex)
Returns a view of the portion of the list between
fromIndex(inclusive) andtoIndex(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
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
5. PriorityQueue (Min-Heap / Max-Heap)
PriorityQueueis 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
Listthat 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 aList).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.