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# 0442. Find All Duplicates in an Array

<https://leetcode.com/problems/find-all-duplicates-in-an-array>

## Description

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`
* `1 <= n <= 105`
* `1 <= nums[i] <= n`
* Each element in `nums` appears **once** or **twice**.

## ac

```java
class Solution {
    public List<Integer> findDuplicates(int[] nums) {
        List<Integer> res = new ArrayList<Integer>();
        // corner cases
        if (nums.length == 0) return res;

        // iterate 
        int n = nums.length;
        for (int i = 0; i < n; i++) {
            while (nums[i] < n && i != nums[i] && nums[nums[i]] != nums[i]) {
                swap(nums, i, nums[i]);
            }
            // special case, n
            if (nums[i] == n) swap(nums, 0, i);
        }

        // final check get result
        if (nums[0] != n) res.add(nums[0]);
        for (int i = 1; i < n; i++) {
            if (nums[i] != i) {
                res.add(nums[i]);
            }
        }

        return res;
    }

    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
}
/* same code as 448*/
```