Is Sum of Unique Elements easy or hard?

Sum of Unique Elements is classified as an Easy problem on LeetCode with a high acceptance rate around 80%. The logic mainly involves counting occurrences using a hash table and filtering values with frequency equal to one.

Sum of Unique Elements Python/Java solution

In Python, a dictionary or collections.Counter can track element frequencies, then sum keys with count equal to 1. In Java, a HashMap stores counts during iteration and a second loop calculates the result.

How to solve Sum of Unique Elements in O(n)?

Traverse the array and store counts in a hash table. After counting, iterate through the map and add numbers whose frequency equals 1. Because both counting and summation steps are linear, the overall complexity remains O(n).

What is the best approach for Sum of Unique Elements?

The most common solution uses a hash map to count frequencies of each number. After building the frequency table in O(n) time, iterate through it and sum values that appear exactly once. This approach is simple, reliable, and optimal for unsorted arrays.

Is Sum of Unique Elements asked at Google/Amazon/Meta?

Frequency counting problems like Sum of Unique Elements commonly appear in coding interviews at companies such as Amazon, Google, and Meta. They test understanding of hash tables, duplicate handling, and efficient linear-time processing.

What data structure is used in Sum of Unique Elements?

The primary data structure is a hash table (or dictionary/map) used to store the frequency of each number. Some solutions also use hash sets to track numbers seen once versus multiple times. Both rely on constant-time average lookups.

What is the time complexity of Sum of Unique Elements?

The optimal solution runs in O(n) time where n is the length of the array. Each element is processed once to build a frequency map, and hash table operations like insert and lookup take constant time on average. Space complexity is O(n) due to storing counts.

1748. Sum of Unique Elements - Practice Online

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