 # Program: Find out duplicate number between 1 to N numbers.

 Description: ```You have got a range of numbers between 1 to N, where one of the number is repeated. You need to write a program to find out the duplicate number.```

 Code: ```package com.java2novice.algos; import java.util.ArrayList; import java.util.List; public class DuplicateNumber { public int findDuplicateNumber(List numbers){ int highestNumber = numbers.size() - 1; int total = getSum(numbers); int duplicate = total - (highestNumber*(highestNumber+1)/2); return duplicate; } public int getSum(List numbers){ int sum = 0; for(int num:numbers){ sum += num; } return sum; } public static void main(String a[]){ List numbers = new ArrayList(); for(int i=1;i<30;i++){ numbers.add(i); } //add duplicate number into the list numbers.add(22); DuplicateNumber dn = new DuplicateNumber(); System.out.println("Duplicate Number: "+dn.findDuplicateNumber(numbers)); } }```

 Output: `Duplicate Number: 22`

 YouTube video: Similar logic on identifying missing number between 1 to N numbers.