PermMissingElem.java

class Solution {
    public int solution(int[] A) {
        int sum = 0;
        int sumA = 0;
        for (int i = 0; i < A.length; i++) {
            int digit = i + 1;
            sum += digit;
            sumA += A[i];
        }
        return A.length + 1 - (sumA - sum);
    }
}