Find Missing Number From Series/Array

• Calculate A = n (n+1)/2 where n is largest number in series 1...N.
• Calculate B = Sum of all numbers in given series
• Missing number = A – B

public class FindMissingNumber {
  public static void main(String[] args) {
    //10 is missing
    int[] numbers = {1,2,3,4,5,6,7,8,9, 11,12};
     
    int N = 12;
    int idealSum = (N * (N + 1)) / 2;
    int sum = calculateSum(numbers);
 
    int missingNumber = idealSum - sum;
    System.out.println(missingNumber);
  }
 
  private static int calculateSum(int[] numbers) {
    int sum = 0;
    for (int n : numbers) {
      sum += n;
    }
    return sum;
  }
}

// Missing element
import java.io.*;
 
class GFG
{
	static int search(int ar[],
					  int size)
	{
		int a = 0, b = size - 1;
		int mid = 0;
		while ((b - a) > 1)
		{
			mid = (a + b) / 2;
			if ((ar[a] - a) != (ar[mid] - mid))
				b = mid;
			else if ((ar[b] - b) != (ar[mid] - mid))
				a = mid;
		}
		return (ar[a] + 1);
	}
	 
	// Driver Code
	public static void main (String[] args)
	{
		int ar[] = { 1, 2, 3, 4, 5, 6, 8 };
		int size = ar.length;
		System.out.println("Missing number: " +
							search(ar, size));
	}
}