top-interview-questions/easy/others/06_missing_number.cc


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class Solution {
public:
    int missingNumber(vector<int>& nums) {
        const size_t expectedSum = nums.size() * (nums.size()+1) / 2;

        size_t sum = 0;
        for (int x : nums) {
            sum += x;
        }

        return expectedSum - sum;
    }
};