LeetCode. Missing numbers

Topics requirements:

Given a, ..., n, n sequence number 0, 1, 2 comprise, find the number 0 .. n does not appear in the sequence.

Example:

Input: [3,0,1]
Output: 2

Code:

class Solution {
public:
    int missingNumber(vector<int>& nums) {
        int n = nums.size();
        if(n == 0) return -1;
        int count = (1 + n) * n / 2;
        for(int i = 0; i < n; i++) {
            count -= nums[i];
        }
        return count;
    }
};