Sum of Unique Elements

LeetCode 1848 | Difficulty: Easy

Easy

Problem Description

You are given an integer array nums. The unique elements of an array are the elements that appear exactly once in the array.

Return the sum of all the unique elements of nums.

Example 1:

Input: nums = [1,2,3,2]
Output: 4
Explanation: The unique elements are [1,3], and the sum is 4.

Example 2:

Input: nums = [1,1,1,1,1]
Output: 0
Explanation: There are no unique elements, and the sum is 0.

Example 3:

Input: nums = [1,2,3,4,5]
Output: 15
Explanation: The unique elements are [1,2,3,4,5], and the sum is 15.

Constraints:

- `1 <= nums.length <= 100`

- `1 <= nums[i] <= 100`

Topics: Array, Hash Table, Counting

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Approach

Hash Map

Use a hash map for O(1) average lookups. Store seen values, frequencies, or indices. The key question: what should I store as key, and what as value?

When to use

Need fast lookups, counting frequencies, finding complements/pairs.

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Solutions

Solution 1: C# (Best: 150 ms)

Metric	Value
Runtime	150 ms
Memory	35.5 MB
Date	2022-01-17

Solution

public class Solution {
    public int SumOfUnique(int[] nums) {
        int[] freq = new int[101];
        foreach(var item in nums)
        {
            freq[item]++;
        }
        int sum = 0;
        for(int i=0;i<101;i++)
        {
            if(freq[i]==1) sum += i;
        }
        
        return sum;
    }
}

📜 1 more C# submission(s)

Submission (2022-01-17) — 153 ms, 36.1 MB

public class Solution {
    public int SumOfUnique(int[] nums) {
        int[] freq = new int[101];
                int sum = 0;
        foreach(var item in nums)
        {
            freq[item]++;
            if(freq[item]==1) sum += item;
            if(freq[item]==2) sum -= item;
        }
        return sum;
    }
}

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Complexity Analysis

Approach	Time	Space
Hash Map	$O(n)$	$O(n)$

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Interview Tips

Key Points

• Start by clarifying edge cases: empty input, single element, all duplicates.
• Hash map gives O(1) lookup — think about what to use as key vs value.
• LeetCode provides 1 hint(s) for this problem — try solving without them first.

💡 Hints

Hint 1: Use a dictionary to count the frequency of each number.