import java.util.*;
class Solution
public List<Integer> findDisappearedNumbers(int[] nums)
Set<Integer> set = new HashSet<>();
for (int num : nums)
set.add(num);
List<Integer> result = new ArrayList<>();
for (int i = 1; i <= nums.length; i++)
if (!set.contains(i))
result.add(i);
return result;
```The provided code is designed to find the missing numbers within a given array `nums` by leveraging a `HashSet` to track the existing numbers. The main idea is to first store all the numbers from the array `nums` into the `HashSet`, effectively creating a set of unique numbers that were present in the array. Then, the code iterates from 1 to the length of the array and checks for each number if it exists in the `HashSet`. If a number is not found in the `HashSet` during this iteration, it implies that the number is missing within the array `nums`, and it is added to the `result` list. Finally, the `result` list, which contains all the missing numbers, is returned as the result of the `findDisappearedNumbers` function.
Here's a breakdown of the code:
1. **Initialization**:
- `Set<Integer> set = new HashSet<>();`: Create a `HashSet` called `set` to store unique numbers from the array `nums`.
- `List<Integer> result = new ArrayList<>();`: Create an empty `ArrayList` called `result` to store the missing numbers.
2. **Loop**:
- `for (int num : nums)`: Iterate through each element `num` in the input array `nums`.
- `set.add(num);`: Add the current `num` to the `HashSet` to track the unique numbers present in `nums`.
3. **Missing Numbers Identification**:
- `for (int i = 1; i <= nums.length; i++)`: Iterate through numbers from 1 to the length of the input array.
- `if (!set.contains(i))`: Check if the current `i` is not present in the `HashSet`. If it's not, it implies that `i` is a missing number.
- `result.add(i);`: Add the missing number `i` to the `result` list.
4. **Return Missing Numbers**:
- The function returns `result`, which contains the list of missing numbers within the input array `nums`.
This code efficiently identifies and collects the missing numbers in the given array using the `HashSet` to track unique numbers and the second loop to identify the missing ones.
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