Find missing number in array in java

We will see how to find missing number in an given array.

Answer is very simple just to remember/recollect our mathematics which we studied in earlier days.

If we know how to find sum of n numbers, answer is simple.

1. First we need to calculate the sum of n numbers. Here n is length of  given array + 1, because 0ne number is missed in given array.

      we can calculate the sum of n numbers using n*(n+1)/2. 

2. Second we need to calculate the sum of all numbers of an array.

Finally we can find missing number in an array as follows:

package com.sample.java.testing;

public class FindMissNumber {
   
    public static void main(String a[]){
       
        int a1[] = {1,3,4,2,6,7,8,9};
        int total = (a1.length+1)*(a1.length+2)/2; ////here n is nothing but arraylength+1
        System.out.println(total);
        int total1 = 0;
        for (int i : a1) {
            total1 = total1 + i;//total of given numbers in an array
        }
        System.out.println("missing number is" + (total - total1));
    }
  
}