Lost number in number sequence in Java

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 0.

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

The solution in Java code

Option 1:

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:

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:

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

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}));
    }
}
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