public class Solution {

    static int ariphmeticSequence(int n) {
        int result = 0;
        int prev = 0;
        while (prev < n) {
            result += prev + 1;
            prev += 1;
        }
        return result;
    }

    public int missingNumber(int[] nums) {
        int temp_summ = 0;
        boolean ifZero = false;
        for(int i: nums) {
            if (i == 0) ifZero = true;
            temp_summ += i;
        }
        if (ariphmeticSequence(nums.length) == temp_summ) {
            if (!ifZero) return 0;
            return nums.length + 1;
        } else {
            return ariphmeticSequence(nums.length) - temp_summ;
        }
    }
}