Concatenation of Array

LeetCode

01 · Question

Given nums of length n, return an array of length 2n containing nums twice: [nums, nums].

02 · Analysis

Allocate the result once, then fill both halves in the same loop:

Write nums[i] to ans[i].
Write the same value to ans[i + n].

The point: use an index offset to map one input position to both output positions.

Complexity: $O(n)$ time and $O(n)$ output space, with $O(1)$ auxiliary space.

03 · Solution

Fill both halves in one pass

1def getConcatenation(nums: List[int]) -> List[int]:
2    n = len(nums)
3    ans = [0] * (2 * n)
4    for i in range(n):
5        ans[i] = nums[i]
6        ans[i + n] = nums[i]
7    return ans

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