Hash Map / Set· 15 min

Unique Number of Occurrences

LeetCode #1207 · Easy · Interactive step-by-step visualization

Setup

Example inputarr = [1,2,2,1,1,3]
Step 1 / 10

Current step

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Count 1. The frequency map groups equal values together. 1 has appeared 1 time(s).

Step log

arr

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count of 1

1

Active cursor
Comparing
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Leaving or removed
unique-number-of-occurrences.java
1public boolean uniqueOccurrences(int[] arr) {
2    Map<Integer, Integer> frequency = new HashMap<>();
3    for (int value : arr)
4        frequency.merge(value, 1, Integer::sum);
5    Set<Integer> used = new HashSet<>();
6    for (int count : frequency.values())
7        if (!used.add(count)) return false;
8    return true;
9}
● Java
Time
O(n)
Space
O(n)
Pattern
hashing
Frames
10
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