When to use a HashMap (Dictionary)?
- Store key-value pairs
- Count frequencies
- Check existence in O(1)
Python syntax: Create: d = {} or d = defaultdict(int)
How do you add/update a value in a HashMap?
- d[key] = value
- d[key] = d.get(key, 0) + 1
This allows for both setting a value and incrementing a count.
What is a common use case for a HashMap?
- Two Sum
- Group Anagrams
- Counting problems
These problems often require efficient key-value storage.
When to use a Set?
- Check uniqueness
- Check presence in O(1)
Python syntax: Create: s = set()
How do you add an element to a Set?
s.add(element)
This method adds an element to the set if it is not already present.
What is a common use case for a Set?
- Finding duplicates
- Tracking seen elements in sliding window
Sets are useful for operations that require uniqueness.
When to use a Stack (List)?
- Need LIFO (Last In First Out)
Python syntax: Create: stack = []
How do you push an element onto a Stack?
stack.append(element)
This adds an element to the top of the stack.
What is a common use case for a Stack?
- Valid Parentheses
- Monotonic stack problems
Stacks are often used in algorithms that require backtracking.
When to use a Queue (Deque)?
- Need FIFO (First In First Out)
Python syntax: Import: from collections import deque
How do you enqueue an element in a Queue?
queue.append(element)
This adds an element to the end of the queue.
What is a common use case for a Queue?
- BFS
- Level-order traversal
- Sliding window maximum
Queues are essential for breadth-first search algorithms.
When to use a Heap (Priority Queue)?
- Access min/max element repeatedly
Python syntax: Import: from heapq import heappush, heappop
How do you create a min heap?
heap = []
This initializes an empty list to be used as a min heap.
What is a common use case for a Heap?
- Top K elements
- Kth largest
- Merge K sorted lists
Heaps are useful for problems that require efficient retrieval of the smallest or largest elements.
When to use a Counter?
- Count frequencies of elements
Python syntax: Import: from collections import Counter
How do you create a Counter from an array?
count = Counter(array)
This initializes a Counter object that counts the occurrences of each element in the array.
What is a common use case for a Counter?
- Top K Frequent Elements
- Anagram problems
Counters simplify frequency counting tasks in various algorithms.
