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Given an integer array nums of length n where all the integers of nums are in the range [1, n] and each integer appears once or twice, return an array of all the integers that appears twice. You must write an algorithm that runs in O(n) time and uses only constant auxiliary space (not counting the space needed to store the output).
Example 1
Input: nums = [4,3,2,7,8,2,3,1]
Output: [2,3]
Explanation: 2 appears at indices 2 and 5, and 3 appears at indices 1 and 6. Every other value appears once.
Example 2
Input: nums = [1,1,2]
Output: [1]
Explanation: 1 appears twice and 2 appears once, so the answer is [1].
Example 3
Input: nums = [1]
Output: []
Explanation: The only value appears once, so there are no duplicates.
For each number, negate the value at the corresponding index. If it's already negative, the number is a duplicate.
Run checks all cases above. Submit evaluates all test cases.