Missing Int O(n)

public class MissingInt {


    public static void main (String args[]) {

        int[] array = {3,2,1,13,5,6,7,8,9,10,11,12,14};

        // sort the list O(nlogn)

        // or

        // sum the numbers O(n)
        // then subtract the numbers O(n)

        int fullSum = array.length + 1;
        int listSum = 0;


        for (int i=0; i<array.length; i++)
        {
            listSum += array[i];
            fullSum += (i+1);

        }

        System.out.println (fullSum-listSum);
    }
}