Unique Number of Occurrences.md

Algorithm

1. Count the number of occurrences of each number in the array. Save these counts in a Map.
2. Check if any of the "values" in the Map are repeated:
   • put all the Map values in a Set
   • compare the sizes of the Map and Set. If they're the same size, then the Map has all unique values.

Solution

class Solution {
    public boolean uniqueOccurrences(int[] nums) {
        if (nums == null) {
            return true;
        }

        Map<Integer, Integer> map =  new HashMap();
        for (int n : nums) {
            map.merge(n, 1, Integer::sum);
        }

        Set<Integer> set = new HashSet(map.values());
        return map.size() == set.size();
    }
}

Time/Space Complexity

• Time Complexity: O(n)
• Space Complexity: O(n)

Links

• github.com/RodneyShag