## The challenge

An ordered sequence of numbers from 1 to N is given. One number might have deleted from it, then the remaining numbers were mixed. Find the number that was deleted.

Example:

• The starting array sequence is `[1,2,3,4,5,6,7,8,9]`
• The mixed array with one deleted number is `[3,2,4,6,7,8,1,9]`
• Your function should return the int `5`.

If no number was deleted from the array and no difference with it, your function should return the int ``.

Note that N may be 1 or less (in the latter case, the first array will be `[]`).

## The solution in Java code

Option 1:

 ``````1 2 3 4 5 6 7 `````` ``````import java.util.stream.IntStream; public class Solution { public static int findDeletedNumber(int[] arr, int[] mixedArr) { return IntStream.of(arr).sum() - IntStream.of(mixedArr).sum(); } } ``````

Option 2:

 `````` 1 2 3 4 5 6 7 8 9 10 `````` ``````import java.util.Arrays; public class Solution { public static int findDeletedNumber(int[] arr, int[] mixedArr) { return Arrays.stream(arr) .filter(arrElement -> Arrays.stream(mixedArr).noneMatch(mixedElement -> arrElement == mixedElement)) .findFirst() .orElse(0); } } ``````

Option 3:

 ``````1 2 3 4 5 6 7 `````` ``````import static java.util.stream.IntStream.of; class Solution { static int findDeletedNumber(int[] arr, int[] mixedArr) { return arr.length == mixedArr.length ? 0 : arr.length * (arr.length + 1) / 2 - of(mixedArr).sum(); } } ``````

## Test cases to validate our solution

 `````` 1 2 3 4 5 6 7 8 9 10 11 12 `````` ``````import org.junit.Test; import static org.junit.Assert.assertEquals; import org.junit.runners.JUnit4; public class SolutionTest { @Test public void basicTests() { assertEquals(2, Solution.findDeletedNumber(new int[]{1,2,3,4,5}, new int[]{3,4,1,5})); assertEquals(5, Solution.findDeletedNumber(new int[]{1,2,3,4,5,6,7,8,9}, new int[]{1,9,7,4,6,2,3,8})); assertEquals(0, Solution.findDeletedNumber(new int[]{1,2,3,4,5,6,7,8,9}, new int[]{5,7,6,9,4,8,1,2,3})); } } ``````