aloalgoFrequency Counting
Frequency counting is one of the most common hash map patterns. It involves counting how many times each element appears, then using those counts to solve the problem.Building a Frequency Map
Common Problems
Top K Frequent Elements
Valid Anagram
First Unique Character
Majority Element
Group Anagrams
Frequency Comparison Patterns
Check if Two Strings Have Same Frequency
Minimum Window Substring
Counter Arithmetic
Tips
- Use Counter: It's cleaner and has useful methods like most_common().
- Counter[missing] = 0: Unlike dict, Counter returns 0 for missing keys.
- Sorting by frequency: sorted(arr, key=freq.get) or Counter.most_common().
- Compare Counters directly: Counter(a) == Counter(b) checks if same frequencies.
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