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home / developersection / forums / given an array of non-duplicating numbers from 1 to n where one number is missing, find it.
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Mukul Goenka
Mukul Goenka

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3 Answers

Aryan Kumar
Aryan Kumar 28 Apr 2023
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o find the missing number in an array of non-duplicating numbers from 1 to n, you can use the following approach:

  1. Calculate the sum of all the numbers from 1 to n using the formula: sum = n * (n + 1) / 2
  2. Calculate the sum of all the numbers in the array
  3. Subtract the sum of the array from the sum of all the numbers from 1 to n. The difference will be the missing number.

Here's a sample code snippet in Java that implements this approach:

public class FindMissingNumber {
   public static int findMissing(int[] arr, int n) {
       int sum = n * (n + 1) / 2;
       int arrSum = 0;
       for (int i = 0; i < arr.length; i++) {
           arrSum += arr[i];
       }
       return sum - arrSum;
   }
   public static void main(String[] args) {
       int[] arr = {1, 2, 4, 5, 6};
       int n = 6;
       int missingNumber = findMissing(arr, n);
       System.out.println("The missing number is: " + missingNumber);
   }
}

In this example, we have an array of numbers from 1 to 6, with one number missing (3). We call the findMissing() method, passing in the array and the value of n (which is the length of the array plus one).

The findMissing() method calculates the sum of all the numbers from 1 to n using the formula sum = n * (n + 1) / 2, then calculates the sum of the array using a loop. The missing number is then calculated by subtracting the sum of the array from the sum of all the numbers from 1 to n.

Krishnapriya Rajeev
Krishnapriya Rajeev 24 Mar 2023
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The code given below shows how to find the missing number in a non-duplicating array of numbers:

class missingNum 
{ 
    public static void main(String[] args)
    {
        int[] nums = { 1, 2, 3, 5, 4, 8, 6};	// Declare and initialize and an integer array.
        int n = nums.length;	// Store length of the integer array
        int sum = ((n + 1) * (n + 2)) / 2;	// Sum of n numbers
        for (int i = 0; i < n; i++)
            sum -= nums[i];		// Subtracts each element from sum
         System.out.println("The missing number is " + sum + ".");
    }
}
OUTPUT:
The missing number is 7.

We can obtain the missing element by subtracting the sum of all elements from the sum of first n natural numbers, where n is the largest number in the array.

Ravi Vishwakarma
Ravi Vishwakarma 22 Nov 2021
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class HelloWorld {
   public static void main(String[] args) {
       int[] array={4,3,8,7,5,2,6};
       int missingNumber = findMissingNum(array);
       System.out.println('Missing Number is '+ missingNumber); 
   }
   public static int findMissingNum(int[] array) {
       int n=array.length+1;
       int sumOfFirstNNums=n*(n+1)/2;
       int actualSumOfArr=0;
       for (int i = 0; i < array.length; i++) {
           actualSumOfArr+=array[i];
       }
       return sumOfFirstNNums-actualSumOfArr;
   }
}

Output.

Missing Number is 1

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