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1. Problem Summary

You are given an integer array nums of length n.

Your task is to build a new array ans of length 2n such that:

ans[i]      = nums[i]
ans[i + n]  = nums[i]

for every index i from 0 to n−1.

In other words, you must concatenate the array with itself:

ans = nums + nums

Example:

Input:  nums = [1,2,1]
Output: [1,2,1,1,2,1]

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2. Key Insights

Insight 1: Output size always exactly double

If nums has n elements, ans always has 2n elements.

Insight 2: Direct index mapping

For each i:

ans[i] = nums[i]
ans[i+n] = nums[i]

Insight 3: No mutation or transformation needed

Values are copied exactly, no computation.

Insight 4: Can be done in one or two loops

Both correct:

• Fill ans while iterating once
• Or copy nums twice

Insight 5: No edge cases beyond length handling

Because nums is guaranteed valid.

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3. Approach

Build a new array and fill positions

Steps:

1. Let n = nums.length
2. Create ans with size 2n
3. Loop through indexes i from 0 to n−1: ans[i] = nums[i] ans[i + n] = nums[i]
4. Return ans

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4. Time and Space Complexity

Time Complexity:

O(N)

We copy each element twice.

Space Complexity:

O(N)

Extra array of size 2n allocated.

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5. Java Solution

class Solution {
    public int[] getConcatenation(int[] nums) {
        int n = nums.length;
        int[] ans = new int[2 * n];

        for (int i = 0; i < n; i++) {
            ans[i] = nums[i];
            ans[i + n] = nums[i];
        }

        return ans;
    }
}

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6. Alternative Java Solution (System.arraycopy)

class Solution {
    public int[] getConcatenation(int[] nums) {
        int n = nums.length;
        int[] ans = new int[2 * n];

        System.arraycopy(nums, 0, ans, 0, n);
        System.arraycopy(nums, 0, ans, n, n);

        return ans;
    }
}

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7. Dry Run Examples

Example 1

Input:

[1,2,1]

n = 3
ans initially size 6

Filling:

ans[0] = 1
ans[1] = 2
ans[2] = 1
ans[3] = 1
ans[4] = 2
ans[5] = 1

Output:

[1,2,1,1,2,1]

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Example 2

Input:

[1,3,2,1]

Output:

[1,3,2,1,1,3,2,1]

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Example 3

Input:

[0]

Output:

[0,0]

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Example 4

Input:

[5,4,8]

Output:

[5,4,8,5,4,8]

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Example 5

Input:

[7,7,7,7]

Output:

[7,7,7,7,7,7,7,7]

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8. Why This Solution Works

• Mapping from nums to ans is direct
• No conditional logic required
• Always produces correct length output
• Works for:
  • single element
  • repeated values
  • large arrays

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9. Common Mistakes

1. Incorrect array length (should be 2 * n)
2. Using concatenation methods intended for strings
3. Forgetting second assignment at index i + n
4. Modifying original nums instead of creating ans
5. Off-by-one loop boundaries