Problem

Given an array of integers arr, return true _if the number of occurrences of each value in the array isunique or _false otherwise.

Examples

Example 1

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Input: arr = [1,2,2,1,1,3]
Output: true
Explanation:  The value 1 has 3 occurrences, 2 has 2 and 3 has 1. No two values have the same number of occurrences.

Example 2

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Input: arr = [1,2]
Output: false

Example 3

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Input: arr = [-3,0,1,-3,1,1,1,-3,10,0]
Output: true

Constraints

  • 1 <= arr.length <= 1000
  • -1000 <= arr[i] <= 1000

Solution

Method 1 – Hash Map and Set

Intuition

Count the occurrences of each value, then check if all occurrence counts are unique using a set.

Approach

  1. Count the frequency of each value in the array.
  2. Check if the set of frequencies has the same size as the list of frequencies.

Code

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import java.util.*;
class Solution {
    public boolean uniqueOccurrences(int[] arr) {
        Map<Integer, Integer> freq = new HashMap<>();
        for (int x : arr) freq.put(x, freq.getOrDefault(x, 0) + 1);
        Set<Integer> seen = new HashSet<>(freq.values());
        return seen.size() == freq.size();
    }
}
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def uniqueOccurrences(arr):
    from collections import Counter
    freq = Counter(arr)
    return len(set(freq.values())) == len(freq)

Complexity

  • ⏰ Time complexity: O(n) — One pass to count, one pass to check uniqueness.
  • 🧺 Space complexity: O(n) — For hash maps and sets.