If you want to practice data structure and algorithm programs, you can go through data structure and algorithm interview questions.

#### Problem :

You are given an integer array containing 1 to n but one of the number from 1 to n in the array is missing. You need to provide optimum solution to find the missing number. Number can not be repeated in the arry.

For example:

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int[] arr1={7,5,6,1,4,2}; Missing numner : 3 int[] arr2={5,3,1,2}; Missing numner : 4 |

#### Solution:

- Find the sum of n number using formula n=n*(n+1)/2
- Find the sum of elements present in given array.
- Substract (sum of n numbers – sum of elements present in the array).

#### Java program to find missing number in an array:

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package org.arpit.java2blog; public class MissingNumberMain { public static void main(String[] args) { int[] arr1={7,5,6,1,4,2}; System.out.println("Missing number from array arr1: "+missingNumber(arr1)); int[] arr2={5,3,1,2}; System.out.println("Missing number from array arr2: "+missingNumber(arr2)); } public static int missingNumber(int[] arr) { int n=arr.length+1; int sum=n*(n+1)/2; int restSum=0; for (int i = 0; i < arr.length; i++) { restSum+=arr[i]; } int missingNumber=sum-restSum; return missingNumber; } } |

When you run above program, you will get below output:

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Missing number from array arr1: 3 Missing number from array arr2: 4 |