# 442.Find-All-Duplicates-in-an-Array

## 442. Find All Duplicates in an Array

## 题目地址

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

## 题目描述

```
Given an array of integers, 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.

Find all the elements that appear twice in this array.

Could you do it without extra space and in O(n) runtime?

Example:
Input:
[4,3,2,7,8,2,3,1]

Output:
[2,3]
```

## 代码

### Approach #1 Turn each number negative

```java
class Solution {
  public List<Integer> findDuplicates(int[] nums) {
        List<Integer> res = new ArrayList();
    for (int i = 0; i < nums.length; i++) {
      int index = Math.abs(nums[i]) - 1;
      if (nums[index] < 0) {
        res.add(Math.abs(index + 1));
      }
      nums[index] = -nums[index];  
    }

    return res;
  }
}
```

### Approach #2 Find All Numbers Disappeared in an Array

```java
class Solution {
  public List<Integer> findDisappearedNumbers(int[] nums) {
      List<Integer> res = new ArrayList();

      for (int num : nums){
        int index = Math.abs(num) - 1;
        if (nums[index] > 0) nums[index] = -nums[index];          
      }

      for (int i = 0; i < nums.length; i++){
        if (nums[i] > 0) res.add(i + 1);      
      }
      return res;
  }
}
```
