[LintCode] Find the Missing Number 寻找丢失的数字

class Solution {
public:
    /**    
     * @param nums: a vector of integers
     * @return: an integer
     */
    int findMissing(vector<int> &nums) {
        // write your code here
        long sum = 0, n = nums.size();
        for (auto &a : nums) {
            sum += a;
        }
        return (int)(n * (n + 1) * 0.5 - sum);
    }
};

class Solution {
public:
    /**    
     * @param nums: a vector of integers
     * @return: an integer
     */
    int findMissing(vector<int> &nums) {
        // write your code here
        int res = 0;
        sort(nums.begin(), nums.end());
        for (int i = 0; i < nums.size(); ++i) {
            res ^= nums[i] ^ (i + 1);
        }
        return res;
    }
};