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Pattern - Frequency Counting - Hash Tables | Data Structures | Repovive

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Data Structures19 sections · 729 units49h total

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Pattern - Frequency Counting

Counting occurrences

The most common hash map pattern: count how many times each element appears.

freq := empty hash map
for each element in array
    if element in freq
        freq[element] := freq[element] + 1
    else
        freq[element] := 1

After this, freq[x] tells you how many times x appeared. Time: $O(n)$ . Space: $O(k)$ where $k$ is the number of distinct elements.