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Product of Array Except Self at Meta — Interview Guide

How to approach Product of Array Except Self in a Meta interview: the optimal approach, what Meta interviewers look for, and company-specific tips.

Updated 2026-09MediumPrefix / Postfix

The problem at Meta

Given an array nums, return an array output such that output[i] equals the product of all elements except nums[i], without using division and in O(n).

Meta uses this problem to assess move fast, ship impact, scale thinking. Expect Meta-specific follow-ups that tie the solution back to production reasoning and the company's engineering bar.

Key concepts

output[i] = (product of everything left of i) × (product of everything right of i).
First pass left-to-right accumulates the running left product.
Second pass right-to-left multiplies in the running right product.

Approach

def product_except_self(nums):
    res = [1] * len(nums)
    left = 1
    for i in range(len(nums)):
        res[i] = left
        left *= nums[i]
    right = 1
    for i in range(len(nums) - 1, -1, -1):
        res[i] *= right
        right *= nums[i]
    return res

What Meta looks for

Coding speed and correctness — produce working, optimal code quickly.
Scale thinking — design for billions of users in the system design round.
Builder mindset — bias toward shipping and measuring.
Behavioral: impact, collaboration, and handling ambiguity.

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