class Solution
{
public:
    int missingNumber(vector<int> &nums) //
    {

        int ans = 0;
        unordered_map<int, int> m;
        for (int i = 0; i < nums.size(); i++)
            m[nums[i]]++;

        for (int i = 0; i <= nums.size(); i++)
        {
            if (!(m.find(i) != m.end()))
            // if(m.count(i)==0)
            {
                ans = i;
            }
        }

        return ans;
    }
};

//In the XOR method we take bitwise XOR of indexes and the number, if indexes and number are present in the process their effect gets cancelled and the extra element is remained . 
class Solution
{
public:
    int missingNumber(vector<int> &nums)
    {

        int n = nums.size();
        int x = 0;

        for (int i = 0; i < n; i++) //
        {
            x ^= i;
            x ^= nums[i];
        }

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

        return x - y;
    }
};