268.MissingNumber.cpp

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