class Solution

{

public:

    int missingNumber(vector<int>& nums)

    {

        nums.push_back(-);

        int len=nums.size();

        for(int i=;i<len;i++)

        {

            while(nums[i]>= && nums[i]!=i)

                swap(nums[i], nums[nums[i]]);

        }

        for(int i=;i<=len;i++)

            if(nums[i]==-)

                return i;

        return -;

    }

};

class Solution

{

public:

    int missingNumber(vector<int>& nums)

    {

        int n=nums.size(), sum = (+n)*n/;

        for(int i:nums)

            sum -= i;

        return sum;

    }

};

class Solution

{

public:

    int missingNumber(vector<int>& nums)

    {

        int result = nums.size();

        for(int i=;i<nums.size();i++)

            result = result^i^nums[i];

        return result;

    }

};