Java Perm Missing Element
import java.util.Arrays;

public class Solution {
    public static int permMissingElement(int[] a) {
        int[] sorted = a.clone();
        Arrays.sort(sorted);

        for (int k = 0; k < sorted.length; k++) {
            if (sorted[k] != k + 1) {
                return k + 1;
            }
        }

        return sorted.length + 1;
    }
}

This uses the expected sum of 1..N+1 and subtracts the actual sum to find the missing value.