Append a Second Copy of an Array
Company: Google
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: easy
Interview Round: Other
### Problem
Given `nums` of length `n`, return a length-`2*n` array consisting of the entire input sequence followed by a second copy in the same order. Thus, for every original index `i`, both result positions `i` and `i+n` contain `nums[i]`.
```cpp
class Solution {
public:
vector<int> getConcatenation(vector<int>& nums);
};
```
### Examples
**Example 1:**
```text
nums = [4,7]
Output: [4,7,4,7]
Explanation: Each half is the original two-entry array.
```
**Example 2:**
```text
nums = [9]
Output: [9,9]
Explanation: The single entry appears once in each half.
```
### Constraints
- `1 <= nums.length <= 1000`.
- `1 <= nums[i] <= 1000`.
### Clarifying Questions
This duplicates the entire sequence rather than placing each repeated value beside itself.
```hint Inspect the structure
Which two result positions correspond to one original index?
```
### Reasoning checkpoints
- Return exactly two complete copies, preserving the order and multiplicity of every input entry.
- Handle the shortest input without confusing concatenation with repeating each value beside itself.
### Optional extensions
How would you concatenate k copies while writing each output position once?
Overview: Append a Second Copy of an Array. Return the concatenation of nums with itself: if its length is n, the result has length 2n and entries i and i+n both equal the original nums[i]. Preserve the original order in each half.