1319. Unique Number of Occurrences

• Difficulty: Easy

• Topics: Hash Table

• Similar Questions:

Problem:

Given an array of integers arr, write a function that returns true if and only if the number of occurrences of each value in the array is unique.

Example 1:

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:

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

Example 3:

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

Constraints:

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

Solutions:

class Solution {
public:
    bool uniqueOccurrences(vector<int>& arr) {
        unordered_map<int, int> freq;
        for (auto& num : arr) {
            ++freq[num];
        }

        unordered_set<int> seen;
        for (auto& entry : freq) {
            if (seen.count(entry.second))   return false;
            seen.insert(entry.second);
        }

        return true;
    }
};