What is the time and space complexity of Implement Array.concat?

Time complexity: O(n + m). Space complexity: O(n + m).

Implement Array.concat

Easy

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Solution Guide: Implement Array.concat

Approach

Create a new result array, copy all elements from the original array, then iterate through each argument. If an argument is an array, spread its elements individually into the result; if it is a scalar value, push it directly. This produces a new array without mutating the originals.

Complexity Analysis

Time: O(n + m)
Space: O(n + m)

Pattern

Array Polyfill

Why It Works

Concat produces a shallow copy that merges arrays and values into a single flat array. It only flattens one level of array arguments, leaving nested arrays intact.

Updated Feb 2026