Hashing and maps in Java

Why it matters

Hash maps are used for counts, grouping, and fast lookup.

Problems to practice

• Valid Anagram
• Top K Frequent Elements
• Subarray Sum Equals K
• First Non-Repeating Character
• Longest Consecutive Sequence

Example: frequency count

Map<Character, Integer> freq = new HashMap<>();
for (char ch : s.toCharArray()) {
    freq.put(ch, freq.getOrDefault(ch, 0) + 1);
}

Interview guidance

Use HashMap when average O(1) lookup is enough. Use TreeMap when order matters.