Balanced Sum
Reported by candidates from JP Morgan's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
JP Morgan's Balanced Sum question hits you with a deceptively simple premise that hides array-manipulation complexity. You're given an array and need to find a partition point where the sum of elements on the left equals the sum on the right, or verify no such point exists. It's been showing up consistently in their June 2025 assessments. The catch isn't the concept; it's handling edge cases and doing it in one clean pass. StealthCoder sits in your corner during the live OA in case the logic fragments under pressure.
Pattern and pitfall
This is a classic prefix-sum problem wearing a different name. You compute the total sum upfront, then iterate through the array tracking a running left sum. At each index, the right sum is total minus left sum minus current element. When left equals right, you've found the balance point. The pitfall candidates hit: off-by-one errors on array boundaries, forgetting to exclude the pivot element itself, or doing two passes when one suffices. The pattern is pure prefix-sum efficiency. During the live assessment, if you freeze on the recurrence, StealthCoder gives you the structure immediately so you stay in flow.
Memorize the pattern. If you can't, run StealthCoder. The proctor sees the IDE. They don't see what's behind it.
You can drill Balanced Sum cold, or you can hedge it. StealthCoder runs invisibly during screen share and surfaces a working solution in under 2 seconds. The proctor sees the IDE. They don't see what's behind it. Made by an engineer who treats the OA as theater. If yours is tonight, you don't have time to grind. You have time to hedge.
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JP Morgan reuses patterns across OAs. Made by an engineer who treats the OA as theater. If yours is tonight, you don't have time to grind. You have time to hedge. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Balanced Sum FAQ
Does the balance point have to be an actual element, or can it be between elements?+
It's typically the index itself. At index i, left sum is elements 0 to i-1, right sum is i+1 to end. Element i itself is excluded from both sides. Check the exact problem wording in your OA for whether index 0 and the last index count as valid answers.
What if no balance point exists?+
Return -1, null, or false depending on the return type specified. JP Morgan assessments usually ask for an index or a boolean. Read the signature carefully. If the whole array is the answer (left and right both empty), that's often valid.
Do I need to handle negative numbers or zeros?+
Yes. Zeros don't break prefix-sum logic. Negatives reduce the running sum but don't change the algorithm. Edge case: array with all zeros. Every index is a valid balance point; return the first one or follow the problem spec.
Can I do this in a single pass?+
Absolutely. One pass to compute total sum, one pass to find the balance point. Two passes, O(n) time, O(1) space. That's the expected solution. Don't use a hash map or build extra arrays unless forced.
Is this still asked at JP Morgan in 2025?+
Yes. Array problems with prefix-sum flavor are evergreen in their assessments, especially for junior and mid-level roles. The name and framing shift, but the core pattern remains. Know this one cold.