Find Every Value That Appears Twice

# Find Every Value That Appears Twice An integer array `nums` has length `n`, all its values lie in `[1,n]`, and every occurring value appears either once or twice. Return all values that appear twice, each listed once. Your algorithm must take `O(n)` time and use constant auxiliary space, excluding the returned result. ### Function `vector<int> findDuplicates(vector<int>& nums)` ### Example 1 ```text Input: nums = [2,5,2,1,5] Output: [2,5] ``` Both `2` and `5` occur twice; `1` occurs once. ### Example 2 ```text Input: nums = [3,1,4,2] Output: [] ``` Every value occurs once. ### Constraints - `n == nums.length` and `1 <= n <= 100000`. - `1 <= nums[i] <= n`. - Every value present appears once or twice. ### Clarifications Return values rather than occurrence positions. The value range is part of the contract, and not every number in `[1,n]` must be present. An auxiliary set whose size grows with the input does not meet the stated space bound. ### What a Strong Answer Covers - Values present once or twice, including values at the range endpoints. - Why every duplicate is identified exactly once. - How the method meets the required time and auxiliary-space bounds. - Any temporary or permanent effects on the supplied array.