442\. Find All Duplicates in an Array

Medium

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 extra space.

**Example 1:**

**Input:** nums = [4,3,2,7,8,2,3,1]

**Output:** [2,3] 

**Example 2:**

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

**Output:** [1] 

**Example 3:**

**Input:** nums = [1]

**Output:** [] 

**Constraints:**

*   `n == nums.length`
*   <code>1 <= n <= 10<sup>5</sup></code>
*   `1 <= nums[i] <= n`
*   Each element in `nums` appears **once** or **twice**.