Arrays5

Q5. An array contains all the integers from 1 to n except for one number which is missing (so there are n-1 elements in array). Write code to find the missing integer.
Answer:

public class Arrays5 {
    public static int findMissing(int[] a) {
        int n = a.length + 1;
        int sum = 0;

        for (int num : a) {
            sum += num;
        }

        int sigmaN = n * (n + 1) / 2;

        return sigmaN - sum;
    }

    public static int findMissing2(int[] a) {
        int n = a.length + 1;
        int missing = 1;
        for (int i = 2; i <= n; i++) {
            missing ^= i;
        }

        for (int num : a) {
            missing ^= num;
        }
        return missing;
    }

    public static void main(String[] args) {
        int[] a1 = { 1, 2, 4, 5, 6, 7 }; // 3 is missing from 1 to 7
        System.out.println(findMissing(a1));

        int[] a2 = { 1, 2, 3, 4, 5, 6, 7 }; // 8 is missing from 1 to 8
        System.out.println(findMissing(a2));

        System.out.println(findMissing2(a1));
        System.out.println(findMissing2(a2));
    }
}