My solution to the https://codility.com/demo/take-sample-test/perm_missing_elem problem. solved in 30ssec flat!

Solution.java

class Solution {
    public int solution(int[] A) {
        int r = 0;
        for (int i = 1; i <= A.length + 1; i++) {
            r = r ^ i;
        }
        for (int i = 0; i < A.length; i++) {
            r = r ^ A[i];
        }
        return r;
    }
}