Missing Number Problem.

MissingNumber.java

public class MissingNumber {
    public static void main(String[] args) {
        MissingNumber missingNum = new MissingNumber();        
        System.out.println("Missing Number : " + missingNum.findMissingNum(new int[] {3,1,2,5,4,7,9,8}, 9));
    }
    
    private int findMissingNum(int[] intArray, int n) {
        int res = 0;
        
        int sum = 0;
        for(int i=0;i<intArray.length;i++) {
            sum += intArray[i];
        }
        
        return ((n * (n + 1))/2) - sum;
    }
}