Return every value that appears twice in an array, sorted in ascending order. Test your code, then compare with the explained solution. Free online practice.

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Valid Sudoku

Return every value that appears twice in an array, sorted in ascending order.

You are given an integer array `nums` of length `n` where every element is in the range `[1, n]`, and each integer appears either once or twice. Return an array of all the integers that appear twice. The returned array must be sorted in **ascending order**.

**Input / output**
- Input: `nums: int[]`
- Output: `int[]` containing every value that appears exactly twice, sorted ascending

## Example
```json
Input: {"nums":[4,3,2,7,8,2,3,1]}
Output: [2,3]
```

## Constraints
- `1 <= nums.length <= 10^5`
- `1 <= nums[i] <= nums.length`
- Each integer in `nums` appears either once or twice

## Follow-up
Can you solve it in `O(n)` time and `O(1)` extra space (not counting the output array), for example by marking visited values in place with `nums[abs(val) - 1] *= -1` and detecting when a position is already negative?

Every number is between `1` and `n`, and each value appears at most twice. That means you can either count what you have seen with extra memory, or reuse the input array itself as a visitation map by jumping to index `value - 1`.

Use the problem guarantees

Scan the array once, track values already seen, and append a value when it appears the second time. Because the final answer must be deterministic and sorted, sort the collected duplicates before returning them.

Warm up with a hash-set solution

For each value `val`, look at index `abs(val) - 1`. If that slot is still positive, negate it to mark that `val` has been seen once. If that slot is already negative, then `val` is appearing for the second time, so it belongs in the answer.

Reuse the array as the visited structure

This is the classic follow-up solution: detect duplicates in one pass by negating `nums[abs(val) - 1]`, collect values that hit an already-negative slot, then sort the output so the returned array is always in ascending order.

Final in-place marking solution