Frequency Counting
Counting frequencies of elements is a classic problem easily solved with HashMap. It's used in many applications: word count, character frequency, finding duplicates, etc.
Example: Count frequency of each number in an array
public static Map countFrequencies(int[] arr) {
Map freq = new HashMap<>();
for (int num : arr) {
freq.put(num, freq.getOrDefault(num, 0) + 1);
}
return freq;
}
Common frequency-based problems:
- Find the element that appears more than n/2 times (Moore's voting).
- Find the element with highest frequency.
- Check if an array contains duplicates.
- Group anagrams (count characters).
Character frequency in a string:
public static Map charFrequency(String s) {
Map freq = new HashMap<>();
for (char c : s.toCharArray()) {
freq.put(c, freq.getOrDefault(c, 0) + 1);
}
return freq;
}
Two Minute Drill
- HashMap simplifies frequency counting.
- getOrDefault() handles missing keys elegantly.
- Common in problems: word count, duplicate detection, anagram check.
- Frequency maps are the foundation for many algorithms.
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