Java solution with explanation [O(n) time and O(n) space] - HashSet

Find duplicate by storing nos in a hashset.

To find the missing no we find the sum of the nos and the difference b/w the sum and the sum of the first n natural nos.

The missing no is the duplicate + difference.

```
public class Solution {
public int[] findErrorNums(int[] nums) {
Set<Integer> set = new HashSet<>();
int repeating = -1; int sum = 0;
for(int num : nums) {
boolean added = set.add(num);
if(!added) {
repeating = num;
}
sum += num;
}
int error = nums.length * (nums.length + 1) / 2 - sum;
int[] result = new int[2];
result[0] = repeating;
result[1] = repeating + error;
return result;
}
}
```