find the missing number

public int (int[] nums) {
    if (nums == null || nums.length == 0)
        return 0;
    int n = nums.length;
    int total = (0 + n) * (n + 1) / 2;
    for (int i : nums)
        total -= i;
    return total;
}

public int (int[] nums) {
    if (nums == null || nums.length == 0)
        return 0;
    int ans = 0, n = nums.length;
    for (int i = 0; i < n; ++i) {
        ans ^= i;
        ans ^= nums[i];
    }
    return ans ^= n;
}