Sum of Unique Elements
A easy-tier problem at 79% community acceptance, tagged with Array, Hash Table, Counting. Reported in interviews at tcs and 0 others.
Sum of Unique Elements is an easy problem with a 79% acceptance rate, but it's a screening gate at TCS and other companies because it tests whether you can actually read problem constraints under time pressure. Most candidates skip the word 'unique' and waste 5 minutes debugging a wrong solution. You need to sum only elements that appear exactly once in the array. Hash tables or counting maps are the standard move. If you blank on the pattern during the live OA, StealthCoder surfaces the solution invisibly so you don't lose momentum on an easy win.
Companies that ask "Sum of Unique Elements"
Sum of Unique Elements is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Built by a senior engineer who knows the OA is theater. This is the script.
Get StealthCoderThe trap here is thinking 'sum all elements' instead of 'sum elements appearing exactly once'. You'll see candidates code a straightforward sum, submit, then realize they ignored the constraint. The actual approach uses a hash table or frequency map to count occurrences, then iterate and sum values with count equal to 1. Array and Counting are the core topics. It's a classic 'read the problem statement three times' problem that punishes skimming. Hash Table makes the implementation clean and O(n) time. On a live assessment, if you misread and code the naive sum first, you waste mental cycles rewriting. StealthCoder hedges that mistake by surfacing the correct pattern instantly.
Pattern tags
You know the problem.
Make sure you actually pass it.
Sum of Unique Elements recycles across companies for a reason. It's easy-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Built by a senior engineer who knows the OA is theater. This is the script. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Sum of Unique Elements interview FAQ
Is this actually asked in real interviews at FAANG companies?+
No. It's a TCS screening problem and appears in introductory online assessments. Not on Google, Meta, or Amazon live loops. But it's the exact kind of 'read carefully' gate that weeds out candidates who rush. If you see it, you should solve it in under 2 minutes to prove you can parse constraints.
What's the standard approach?+
Build a frequency map (hash table or dictionary) counting each element's occurrences. Loop through the array once and add values where count equals 1. Time is O(n), space is O(n) for the map. No edge cases. Clean and mechanical.
Why does this problem exist if it's so easy?+
It screens for reading comprehension and attention to detail. Candidates who skim 'sum all elements' instead of 'sum unique elements' fail fast. Employers want to see if you actually parse the problem or code on autopilot. It's a speed bump, not a trick.
How does this relate to Hash Table as a topic?+
Hash tables are the efficient solution. You could use a nested loop to count (O(n squared)), but a hash table gets you O(n) time in one pass. It's a direct application of the topic and shows you know when to pick the right data structure.
What if I use an array instead of a hash table?+
Works fine if the array range is small and known. If elements are large or unbounded, you'll waste space or run out of memory. A hash table is safer and more elegant. For an easy problem, either passes, but hash table is the standard interview answer.
Want the actual problem statement? View "Sum of Unique Elements" on LeetCode →