Java Program To Find Out Duplicate Number Between 1 to N

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import java.util.ArrayList;
import java.util.List;

public class JavaDuplicateNumber {
	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(20);
		JavaDuplicateNumber dn = new JavaDuplicateNumber();
		System.out.println("Duplicate Number: " + dn.findDuplicateNumber(numbers));
	}
}
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