How to find the missing number in a given Array from number 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