Is Concatenation of Array easy or hard?

Concatenation of Array is categorized as an Easy problem on LeetCode with a very high acceptance rate. It focuses on fundamental array operations and is often used as a beginner exercise for iteration and indexing.

Concatenation of Array Python/Java solution

In Python, you typically create a list of size 2 * n and assign values using indices or simply use list concatenation like nums + nums. In Java, allocate an int array of size 2 * nums.length and copy elements using a loop.

How to solve Concatenation of Array in O(n)?

Create a new array of length 2n and iterate through nums once. For every index i, assign ans[i] = nums[i] and ans[i + n] = nums[i]. This builds both halves of the result during the same pass, giving linear time complexity.

What is the best approach for Concatenation of Array?

The direct copy approach is the best solution. Allocate a result array of size 2n and copy each element from nums into positions i and i + n during a single loop. This runs in O(n) time and uses O(n) extra space, which is optimal for this problem.

Is Concatenation of Array asked at Google/Amazon/Meta?

Concatenation-style array construction problems appear frequently in coding screens at companies like Amazon and Meta as warm‑up questions. They test basic array manipulation, indexing, and understanding of time and space complexity.

What data structure is used in Concatenation of Array?

The primary data structure is an array. Some implementations also use dynamic lists such as Python lists, Java ArrayList, or C++ vectors to build the result before returning it.

What is the time complexity of Concatenation of Array?

The optimal solution runs in O(n) time where n is the length of the input array. Each element is processed once and copied into two positions in the result array. Space complexity is O(n) because the output array contains 2n elements.

Description Hints 1 Solution Video 4:00

1929. Concatenation of Array

Easy90.2% acceptance7 companies

Given an integer array nums of length n, you want to create an array ans of length 2n where ans[i] == nums[i] and ans[i + n] == nums[i] for 0 <= i < n (0-indexed).

Specifically, ans is the concatenation of two nums arrays.

Return the array ans.

Example 1:

Input: nums = [1,2,1]
Output: [1,2,1,1,2,1]
Explanation: The array ans is formed as follows:
- ans = [nums[0],nums[1],nums[2],nums[0],nums[1],nums[2]]
- ans = [1,2,1,1,2,1]

Example 2:

Input: nums = [1,3,2,1]
Output: [1,3,2,1,1,3,2,1]
Explanation: The array ans is formed as follows:
- ans = [nums[0],nums[1],nums[2],nums[3],nums[0],nums[1],nums[2],nums[3]]
- ans = [1,3,2,1,1,3,2,1]

Constraints:

n == nums.length
1 <= n <= 1000
1 <= nums[i] <= 1000

Asked at 7 companies

GE Healthcare

Google Amazon Microsoft Meta Bloomberg

Infosys

Topics

Array Simulation

Hints open one at a time

Hint 1

Build an array of size 2 * n and assign num[i] to ans[i] and ans[i + n]

4:00

Still stuck?Read 3 approaches, or watch a walkthrough.

Open solution

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1class Solution:
2    def getConcatenation(self, nums: List[int]) -> List[int]:
3        

Starter codePython 3Spaces: 4

Examples

Case 1Case 2

Input

[1,2,1]

Input

[1,3,2,1]

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