Leetcode Missing Number problem solution

class Solution(object):
    def missingNumber(self, nums):

        L = len(nums)
        final = (1+L)*L/2
        for i in nums:
            final -= i
        return final

public int missingNumber(int[] nums) {
        int sum = nums.length * (nums.length + 1) / 2;
        int series = 0;
        for (int num : nums)
            series += num;
        return sum - series;
    }

class Solution {
public:
    int missingNumber(vector<int>& nums)
    {
        int n=nums.size();
        set<int> s;
        for(int i=0;i<n;i++)
        {
            s.insert(nums[i]);
        }
        for(int i=0;i<=n;i++)
        {
            if(s.find(i)==s.end())
                return i;
        }
        return -1;
    }
};

int missingNumber(int* nums, int numsSize) {
    int *result = calloc(numsSize, sizeof(int));
    for(int i = 0; i < numsSize; i++){
        result[nums[i]] = -1;
    }
    for(int i = 0; i < numsSize; i++){
        if(result[i] >= 0) return i;
    }
    return numsSize;
}