Missing Number

Given an array containing n distinct numbers taken from 0, 1, 2, ..., n, find the one that is missing from the array.

For example,
Given nums = [0, 1, 3] return 2.

Note:
Your algorithm should run in linear runtime complexity. Could you implement it using only constant extra space complexity? 

class Solution {
public:
    int missingNumber(vector<int>& nums) {
        int sum = 0, n = nums.size();
        for (auto &a : nums) {
            sum += a;
        }
        return 0.5 * n * (n + 1) - sum;
    }
};

class Solution {
public:
    int missingNumber(vector<int>& nums) {
        int res = 0;
        for (int i = 0; i < nums.size(); ++i) {
            res ^= (i + 1) ^ nums[i];
        }
        return res;
    }
};