4 lines C++ Solution N*(N+1) /2

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    I tried the XOR method also but this one is simple and I think easier (maybe not better) to understand. Basically if you want to count every number from 1 to 100 you use the formula n * (n+1)/2. So What I did is sum the values in the vector and subtract that sum from the formula I stated based on nums.size().

        int missingNumber(vector<int>& nums) 
            int sum = 0;
            for(auto i : nums)
                sum += i;
        return nums.size() * (nums.size() + 1) / 2 - sum;

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    This soulution work normally only when there is only one missing number in the array.
    Think about array: [1,3,5].

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    Yes you are totally right. I was aware of that. Since the problem only stated one is missing I tried the above code and passed all cases. If more than one was indeed missing this option would not have worked.

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