Find out duplicate number between 1 to N numbers Java Programs

Q: 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<Integer> numbers){
        
        int highestNumber = numbers.size() - 1;
        int total = getSum(numbers);
        int duplicate = total - (highestNumber*(highestNumber+1)/2);
        return duplicate;
    }
    
    public int getSum(List<Integer> numbers){
        
        int sum = 0;
        for(int num:numbers){
            sum += num;
        }
        return sum;
    }
    
    public static void main(String a[]){
        List<Integer> numbers = new ArrayList<Integer>();
        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