Find Missing Number in an Array from 1 to n in Java

If a single number is missing in an integer array that contains a sequence of numbers values, you can find it basing of the sum of numbers or, basing on the xor of the numbers.

Based on the sum of the numbers −

• The sum of n sequential numbers will be [n*(n+1)]/2. Using this get the sum of the numbers the n numbers.
• Add all the elements in the array.
• Subtract the sum of the numbers in the array from the sum of the n numbers.

Example

import java.util.Scanner;
public class MissingNumber {
   public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);
      System.out.println("Enter the n value: ");
      int n = sc.nextInt();
      int inpuArray[] = new int[n];
      System.out.println("Enter (n-1) numbers: ");
      for(int i=0; i<=n-2; i++) {
         inpuArray[i] = sc.nextInt();
      }
      //Finding the missing number
      int sumOfAll = (n*(n+1))/2;
      int sumOfArray = 0;
      for(int i=0; i<=n-2; i++) {
         sumOfArray = sumOfArray+inpuArray[i];
      }
      int missingNumber = sumOfAll-sumOfArray;
      System.out.println("Missing number is: "+missingNumber);
   }
}

Output

Enter the n value:
5
Enter (n-1) numbers:
1
2
4
5
Missing number is: 3

Using XOR operation − Another way to find the missing number is using XOR.

• Find the XOR of all the numbers up ton.
• Find the XOR of all the numbers in the array.
• Then find the XOR of both results.

Example

import java.util.Scanner;
public class MissingNumber {
   public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);
      System.out.println("Enter the n value: ");
      int n = sc.nextInt();
      int inpuArray[] = new int[n];
      System.out.println("Enter (n-1) numbers: ");
      for(int i=0; i<=n-2; i++) {
         inpuArray[i] = sc.nextInt();
      }
      //Finding the missing number
      int xorArray = inpuArray[0];
      //XOR of elements of the array
      for(int i=1; i<=n-1; i++) {
         xorArray = xorArray ^ i;
      }
      int xorAll = inpuArray[0];
      //XOR of elements of the array
      for(int i=1; i<=n+1; i++) {
         xorAll = xorAll ^ i;
      }
      int missingNumber = xorArray ^ xorAll;
      System.out.println("Missing number is: "+missingNumber);
   }
}

Output

Enter the n value:
5
Enter (n-1) numbers:
1
2
4
5
Missing number is: 3