class Solution {
public:
    int missingNumber(vector<int>& nums) {
        int size = nums.size();
        
        int expect_sum = size * (size+1) / 2;
        
        int actual_sum = 0;

        for(int i = 0; i < size; i++) {
            actual_sum += nums[i];
        }

        return expect_sum - actual_sum;
    }
};