01-arrays-and-hashing

01 · Arrays & Hashing

Trade space for time — a HashMap/HashSet turns O(n²) scans into O(n).

🧠 Intuition

Most array problems that feel like "check every pair" can be reduced to a single pass by remembering what you've seen in a hash structure. Lookups are O(1).

🕵️ When to reach for it

• "Have I seen this value before?" → HashSet
• "What index/count maps to this value?" → HashMap
• Counting frequencies, grouping, deduping.

🧩 Template

Map<Integer, Integer> seen = new HashMap<>();
for (int i = 0; i < nums.length; i++) {
    if (seen.containsKey(want(nums[i]))) { /* found */ }
    seen.put(nums[i], i);
}

📝 Problems

Problem	Difficulty	Solution
Contains Duplicate	🟢	todo
Valid Anagram	🟢	todo
Two Sum	🟢	TwoSum.java
Group Anagrams	🟡	todo
Top K Frequent Elements	🟡	todo
Product of Array Except Self	🟡	todo
Longest Consecutive Sequence	🟡	todo