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.